Magnetic oxide and process for producing same

ABSTRACT

A magnetic oxide obtained by mixing a given amount ranging from 0.20 to 76.0% by weight of at least one member selected from among Ca, Ti, Sn, Si and Mg magnetization control (regulation) materials with hematite, a mixture of hematite and magnetite, or magnetite as a starting material to obtain a mixed powder, adding 0.1 to 4.0% by weight of a liquid or powder substance having a carbon-to-carbon single or double bond to the mixed powder, stirring the mixture to obtain a substantially homogeneous mixture and sintering the obtained mixture in an inert gas at 550° to 1500° C., and a process for producing the same.

TECHNICAL FIELD

The present invention relates to a magnetic oxide suitable for use in awide range of technical fields, e.g., those of toners and carriers forelectrophotography, electric resistance elements and magnetic fluids,which magnetic oxide is produced by mixing a magnetization controlmaterial of Ca, Ti, Sn, Si or Mg with hematite, hematite+magnetite, ormagnetite to obtain a mixed powder and sintering the mixed powder sothat it exhibits a given saturation magnetization. Also, the presentinvention relates to a process for producing the above magnetic oxide.

BACKGROUND ART

A single phase magnetite powder as one of the magnetic oxides has beenutilized in a wide range of technical fields, e.g., those of magneticfluids, electric resistance elements and toners and carriers forelectrophotography. It is highly desired in the art to mass-produce asingle phase magnetite powder exhibiting any predetermined saturationmagnetization at a lowered cost.

The following three general methods are known for producing such amagnetite, and it is know that they have respective drawbacks asdescribed below.

(1) Wet Process: an aqueous solution of Fe²⁺ +2Fe⁺³ is rendered alkalineand coprecipitated.

Drawbacks: production cost is high and it is not suitable for massproduction, although fine particles each having a relatively high purityare readily obtained by coprecipitation.

(2) Dry Process: hematite is heated in either hydrogen and carbonmonoxide or steam to thereby be reduced.

Drawbacks: the reaction is performed in a relatively highly reducingatmosphere, e.g., either high pressure steam or hydrogen and carbonmonoxide, so that danger accompanies the mass production, and that largescale equipment is required.

(3) Process in which natural magnetite is pulverized.

Drawbacks: a natural mineral is used as a starting material, so that itis difficult to stably produce fine quality powder for a prolongedperiod of time.

The magnetite powders produced by the above conventional processes havea drawback in that their saturation magnetization values are so high ascompared with those of the conventional spinel ferrite that it isunfeasible to attain control of the saturation magnetization bycomposition regulation with the result that the conventional magnetitepowders cannot be employed in use in which the saturation magnetizationvalues inherent in magnetite powder are unacceptable. Magnetite powderhaving a saturation magnetization value inherent therein (fixed valuespecified in the second experimental example described later, e.g., upto 92 emu/g) has only partly been employed in use in carriers forelectrophotographic development and the like. Moreover, the abovemagnetite powder having the saturation magnetization value inherenttherein as it is cannot replace ferrite or the like in uses in which ithas been employed, and, when the replacement is to be made, it isrequired to alter the circuit or device to be employeddisadvantageously.

Taking into account the advantages and disadvantages of the aboveprocesses, various proposals have been made with respect to theproduction of magnetite powder for use in magnetic toners forelectrophotography, etc.

For example, Japanese Patent Publication Nos. 238,580/1987, 39,498/1990and 51,505/1990 disclose processes for producing magnetite useful as acarrier component of an electrophotographic developer, especially atwo-component developer composed of a toner and a carrier. Each of theseprocesses comprises providing magnetite powder (or globular magnetiteparticles) as a starting material, granulating the same into globulargrains through, for example, mixing with a binder, followed by heating(sintering), and applying a resin coating to the resultant globularmagnetite particles according to the conventional technique.

The use of a material called soft ferrite as carrier particles is known(U.S. Pat. No. 3,929,657, etc.). In this connection, it is known thatthe carrier particles each composed of the above ferrite not only areexcellent in magnetic properties but also do not require a resin coatinglayer, so that their durability is excellent. Nothing that ferriteparticles have resistance variations depending on the control ofsintering atmosphere, even with the same composition, attempts have beenmade to enlarge resistance variation ranges by changing the sinteringatmosphere. As an example thereof, there can be mentioned an attemptdescribed in Japanese Patent Publication No. 37782/1987. This comprisesproviding an Mg ferrite as a starting material and increasing the amountof ferric oxide (Fe₂ O₃) to more that 53% by mole so as to enlarge itsresistance variation range.

DISCLOSURE OF THE INVENTION

The Inventor's primary object, in view of the above prior art, is toeasily mass-produce a magnetic oxide, in particular, a magnetic oxideexhibiting any desired saturation magnetization at a lowered cost bymixing a magnetization control material of Ca, Ti, Sn, Si or Mg withhematite, hematite+magnetite, or magnetite to obtain a mixed powder,further mixing a substance having a carbon-to-carbon single or doublebond with the mixed powder and sintering the obtained mixture.

Another object of the present invention is to provide a process formass-producing the above magnetic oxide of fine quality at a loweredcost and safely by simple equipment and operation, as compared with theabove conventional processes.

A further object of the present invention is to provide a process formass-producing the above magnetic oxide having any desired saturationmagnetization value.

According to the present invention, there is provided a magnetic oxideobtained by mixing a given amount ranging from 0.20 to 76.0% by weightof at least one member selected from among Ca, Ti, Sn, Si and Mgmagnetization control (regulation) materials with hematite,hematite+magnetite, or magnetite as a starting material to obtain amixed powder, adding 0.1 to 4.0% by weight of a liquid or powderysubstance having a carbon-to-carbon single or double bond to the mixedpowder, stirring the mixture to obtain a substantially homogeneousmixture and sintering the obtained mixture in an inert gas at 550° to1500° C., and also provided is a process for producing the same.

In the above magnetic oxide and the process for producing the same,preferred conditions for embodying the present invention are as setforth below with respect to each magnetization control material.

In the employment of Ca compound

(1) Starting material: hematite or hematite+magnetite

Ca compound: 1.43 to 64.0% by weight (in terms of Ca)

Sintering temperature: 1200° to 1450° C.

(2) Starting material: magnetite

Ca compound: 1.43 to 64.0% by weight (in terms of Ca)

Sintering temperature: 550° to 1450° C.

In the employment of Ti compound

(3) Starting material: hematite+magnetite

Hematite: 0.0 to 79.0% by weight

Magnetite: 9.8 to 98.8% by weight

Ti compound: 1.2 to 52.7% by weight (in terms of Ti) Sinteringtemperature: 1200° to 1450° C.

(4) Magnetite: 47.3 to 98.8% by weight

Ti compound: 1.2 to 52.7% by weight

Sintering temperature: 550° to 1450° C.

In the employment of Sn compound

(5) Hematite or hematite+magnetite: 24.0 to 99.2% by weight

Sn compound: 0.8 to 76.0% by weight

Sintering temperature: 1200° to 1450° C.

(6) Magnetite: 24.0 to 99.2% by weight

Sn compound: 0.8 to 76.0% by weight

Sintering temperature: 550° to 1450° C.

In the employment of Si compound

(7) Hematite (or hematite+magnetite): 35 to 99.5% by weight

Si compound: 0.5 to 65% by weight

Sintering temperature: 1200° to 1450° C.

(8) Magnetite: 35 to 99.5% by weight

Si compound: 35 to 99.5% by weight

Sintering temperature: 550° to 1450° C.

In the employment of Mg compound

(9) Starting compound: hematite (or hematite+magnetite)

Mg compound: 0.20 to 18.50% by weight

Sintering temperature: 550° to 1500° C.

(10) Starting compound: magnetite

Mg compound: 0.20 to 26.20% by weight

Sintering temperature: 550° to 1500° C.

(11) Starting compound: hematite (or hematite+magnetite)

Addition of manganese: 0.20 to 10.0% by weight

Mg compound: 0.20 to 26.20% by weight

Sintering temperature: 550° to 1500° C.

(12) Starting compound: magnetite

Addition of manganese: 0.20 to 10.0% by weight

Mg compound: 0.20 to 26.20% by weight

Sintering temperature: 550° to 1500° C.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an explanatory view of the steps taken in the first embodimentof the present invention;

FIG. 2 is a view of an exemplary heating curve suitable for sintering inthe first embodiment;

FIG. 3 is an explanatory view of the saturation magnetization referredto in the present invention;

FIG. 4 is an explanatory view of the steps taken in the secondembodiment of the present invention;

FIG. 5 is a view of an exemplary heating-cooling curve suitable for thesecond embodiment of the present invention;

FIG. 6 is an explanatory view of the steps taken in the third embodimentof the present invention;

FIG. 7 is a view of an exemplary heating-cooling curve suitable for thethird embodiment of the present invention;

FIG. 8 is an explanatory view of the steps taken in the fourthembodiment of the present invention;

FIG. 9 is a view of an exemplary heating-cooling curve suitable for thefourth embodiment of the present invention;

FIG. 10 is an explanatory view of the steps taken in the fifthembodiment of the present invention;

FIGS. 11(a), 11(b) and 11(c) are explanatory view of the ranges ofsaturation magnetization control attained in the fifth embodiment of thepresent invention;

FIGS. 12(a) and 12(b) are explanatory views of the grain strengthsattained in the fifth embodiment of the present invention; and

FIG. 13 is a view of an exemplary heating curve suitable for the fifthembodiment of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Preferred embodiments of the present invention will now be describedwith reference to the following Experimental Examples.

First Embodiment

Referring to FIG. 1, hematite, hematite+magnetite, or magnetite isblended with 1.43 to 64.0% by weight of Ca to form a mixed powder in theblending step 1. The term "Ca" is employed herein for simplification ofthe description of the specification, and means the Ca moiety, expressedin % by weight, of organic and inorganic compounds each containing Ca,such as CaO, CaCO₃, Ca(OH)₂, CaCl₂ and CaF₂. The sum of the weightpercentage of hematite, hematite+magnetite, or magnetite and the 1.43 to64.0% by weight of this Ca is 100% by weight (that is, for example, theweight percentage of the "O" moiety of CaO is not contained n the valueof 100% by weight). The terminology "hematite+magnetite" employed hereinmeans a mixture (mixed powder) of hematite and magnetite. In particular,the magnetite as a starting material may be either of:

(1) magnetite powder (produced in the house or purchased from othercompanies), and

(2) magnetite powder obtained by pulverizing a product having anoff-specification particle size (recovered item) so as for the same tohave the given particle size.

The product grains described later are those each composed of, forexample, 10³ to 10⁷ starting powder particles (1-3 μm) gathered into aglobular form having a diameter of 50 to 100 μm. Thus, any once producedmagnetite powder product (off-specification item) can be pulverized tothereby easily obtain the desired starting material powder.

In the mixing step 2, the above mixed powder is mixed with 0.1 to 4.0%by weight of a compound having --C--C-- or --C═C-- in its molecules(liquid or solid substance). For example, 2% by weight of PVA (polyvinylalcohol) as a binder and 1% by weight of a polycarboxylate as adispersant are added to hematite powder as a starting material, andfurther water is added for granulation into globular grains. The amountof waster added is in the range of 30 to 70% by weight. When the amountof water was less that 30% by weight, the viscosity of a slurry obtainedby milling was too high to be granulated into globular grains. On theother hand, when the amount of waster exceeded 70% by weight, the slurryconcentration was too low to be granulated into dense globular grains.

In the wet-milling step 3, the mixture obtained in the mixing step 2 iswetly crushed in an attrition mill to thereby prepare a slurry having amixed powder concentration of about 50% by weight.

In the granulating step 4, globular grains are formed. In this step, theslurry is agitated in an attrition mill for 1 hr, and subjected tohot-air drying by a spray dryer to obtain globular grains.

In the sintering step 5, the grains obtained in the granulating step 4are heated at 1200° to 1450° C., provided that those of magnetite may beheated at 550° to 1450° C., in an atmosphere of an inert gas such asnitrogen gas for 2 hr to thereby obtain powder in which single phasemagnetite and another magnetic and/or nonmagnetic phase are coexistent.The value of saturation magnetization of the powder can be regulated bythe amount of Ca added, so that it is feasible to produce a powderymagnetic oxide having any desired saturation magnetization by changingthe amount of Ca added (see Tables 1 to 4). When hematite is present inpart of the hematite or magnetite powder, during the 1200° to 1450° C.sintering step 5, not only is the thermal transition from hematite tomagnetite effected in the inert gas (in weakly reducing atmosphere) butalso the organic substance mixed is brought by the heating in the inertgas into the state of incomplete combustion, in which the hematite isdeprived of oxygen during the thermal decomposition of the organicsubstance to thereby be reduced to markedly promote the conversion tomagnetite. In particular, with respect to magnetite, a magnetic oxidehaving any desired saturation magnetization can be obtained by thesintering at temperatures as relatively low as 550° to 1450° C. This isbecause there is no need for newly making conversion into magnetite(reduction of hematite), so that the strength required for handling,etc. can be obtained by bonding or mild sintering of the startingmaterial magnetite in the form of a vast plurality of grains.

In the disintegration step 6, the powder in which single phase magnetiteand nonmagnetic phase are coexistent, obtained in the sintering step 5,is crushed into a product. According to the above steps, hematite,hematite+magnetite, or magnetite is mixed with the above Ca to obtain amixed powder, and this mixed powder is further mixed with a compoundhaving --C--C-- or --C═C-- in its molecules (liquid or solid substance)and water, milled well, hot-air dried, granulated into globular grainsand sintered in an inert gas at 1200° to 1450° C., provided that thesintering of magnetite is conducted at 550° to 1450° C., so that thepowder in which magnetite and nonmagnetic phase are coexistent (magneticoxide powder) can be produced. Thus, it has become feasible to safelymass-produce the magnetic oxide powder having any desired saturationmagnetization at a lowered cost. Descriptions wilt be made in orderbelow.

Table 1 shows sintering experiment results (hematite) of the presentinvention. In the experiment, Ca was blended with hematite powder in theamount specified in the Table, and mixed with water to obtain a slurryhaving a powder concentration of 50% by weight. The slurry was agitatedin an attrition mill for 1 hr, and dried at 110° C. 1.0% by weight ofpolyvinyl alcohol was added to the obtained powder. The mixture wasmixed in a mortar, and sifted through a 425-μm-mesh standard sieve toobtain grains. 0.5 g of the resultant grains were put in a cylindricalmold having a diameter of 12.5 mm, and shaped under a molding pressureof 1 t/cm². The molding was heated at 1100° to 1500° C. for 2 hr innitrogen gas. The saturation magnetization of each sample after theheating was measured by means of an vibrating sample magnetometer.

(1) When Ca was not added (Sample Nos. 9, 17 and 25) and the conversionto single phase magnetite was effected, the saturation magnetization ofeach of the samples was 92 emu/g.

(2) When the heating was conducted at 1100° C., the conversion tomagnetite was incomplete so as for hematite (α-Fe₂ O₃) to remain withthe result that the saturation magnetization values of Sample Nos. 1 to8 were 80 emu/g or less to show the inappropriateness of the heatingtemperature of 1100° C. On the other hand, when the heating wasconducted at 1500° C., the conversion to magnetite was incomplete so asto form wustite (FeO) with the result that the saturation magnetizationvalues of Sample Nos. 33 to 40 were as low as 87 emu/g or less to showthe inappropriateness of the heating temperature of 1500° C. Therefore,it has been found that the appropriate heating temperature is in therange of 1200° to 1450° C., the amount of Ca added was increased from 0%by weight. Sample Nos. 11, 19 and 27 of Table 1 containing Ca in anamount of 1.43% by weight exhibited a slight change in saturationmagnetization as compared with the sample not containing Ca. The abovevalue of 1.43% by weight was taken as the lower limit of the amount ofCa to be added. On the other hand, the upper limit of the amount of Cato be added was taken as 10.65% by weight at which the saturationmagnetization attributed to the nonmagnetic phase formed by the amountof Ca increased from 1.43% by weight was 20 to 10 emu/g or which wasjust below the value at which melting would occur during the sintering.Therefore, the amount of Ca to be added to hematite was appropriately inthe range of 1.43 to 10.65% by weight.

It has been found from the above experimental results that powder(magnetic oxide) having an arbitrary saturation magnetization in whichmagnetite and nonmagnetic phase are coexistent can be formed bysintering at 1200° to 1450° C. for 2 hr a mixed powder obtained bymixing 1.43 to 10.65% by weight of Ca with hematite.

Table 2 shows sintering experiment results regarding hematite+magnetite.In the experiment, Ca was blended in the amount specified in the Tablewith powder of 1:1 mixture of hematite and magnetite, and treated andmeasured in the same manner as described above with respect to Table 1.

(1) When Ca was not added (Sample Nos. 9, 17 and 25), the saturationmagnetization of each of the samples was 92 emu/g.

(2) When the heating was conducted at 1100° C., the conversion tomagnetite was incomplete so as for hematite (α-FE₂ O₃) to remain withthe result that the saturation magnetization values of Sample Nos. 1 to8 saturation magnetization values of Sample Nos. 1 to 8 were 67 emu/g orless to show the inappropriateness of the heating temperature of 1100°C. On the other hand, when the heating was conducted at 1500° C., theconversion to magnetite was incomplete so as to form wustite (FeO) withthe result that the saturation magnetization values of Sample Nos. 33 to40 were as low as 87 emu/g or less to show the inappropriateness of theheating temperature of 1500° C. Therefore, it has been found that theappropriate heating temperature is in the range of 1200° to 1450° C.

(3) while the heating was conducted at a temperature ranging from 1200°to 1450° C., the amount of Ca added was increased from 0% by weight.Sample Nos. 11, 19 and 27 of Table 2 containing Ca in an amount of 1.43%by weight exhibited a slight change in saturation magnetization ascompared with the sample not containing Ca. The above value of 1.43% byweight was taken as the lower limit of the amount of Ca to be added. Onthe other hand, the upper limit of the amount of Ca to be added wastaken as 10.65% by weight at which the saturation magnetizationattributed to the nonmagnetic phase formed by the amount of Ca increasedfrom 1.43% by weight was 20 to 10 emu/g or which was just below thevalue at which melting would occur during the sintering. Therefore, theamount of Ca to be added to hematite+magnetite was appropriately in therange of 1.43 to 10.65% by weight.

It has been found from the above experimental results that powder(magnetic oxide) having an arbitrary saturation magnetization in whichmagnetite and nonmagnetic phase are coexistent can be formed bysintering at 1200° to 1450° C. for 2 hr a mixed powder obtained bymixing 1.43 to 10.65% by weight of Ca with hematite+magnetite.

Table 3 shows sintering experiment results regarding magnetite. In theexperiment, Ca was blended with magnetite powder in the amount specifiedin the Table, and treated and measured in the same manner as describedabove with respect to Table 1.

(1) When Ca was not added (Sample Nos. 13, 25, 37, 49, 61 and 73), thesaturation magnetization of each of the samples was 91 to 92 emu/g.

(2) When the heating was conducted at 500° C., the magnetite wasconverted to hematite (α-FE₂ O₃) with the result that the saturationmagnetization values of Sample Nos. 1 to 12 were as low as 86 emu/g toshow the inappropriateness of the heating temperature of 500° C. Thecause of the phenomenon of the above conversion has been found to be thepartial oxidation of the magnetite to hematite by a trace (for example,on the order of 10 ppm) of inevitable O₂ contained in the inert gas(nitrogen gas in this embodiment). On the other hand, when the heatingwas conducted at 1500° C., the magnetite was converted to wustite (FeO)with the result that the saturation magnetization values of Sample Nos.85 to 96 were as low as 86 emu/g or less to show the inappropriatenessof the heating temperature of 1500° C. Therefore, it has been found thatthe appropriate heating temperature is in the range of 550° to 1450° C.

(3) While the heating was conducted at a temperature ranging from 550°to 1450° C., the amount of Ca added was increased from 0% by weight.Sample Nos. 15, 27, 39, 51 63 and 75 of Table 3 containing Ca in anamount of 1,434 by weight exhibited a slight change in saturationmagnetization as compared with the sample not containing Ca. The abovevalue of 1.43% by weight was taken as the lower limit of the amount ofCa to be added. On the other hand, the upper limit of the amount of Cato be added was taken as 64.0% by weight at which the saturationmagnetization attributed to the nonmagnetic phase formed by the amountof Ca increased from 1.43% by weight was as low as 20 to 10 emu/g asobserved with respect to Sample Nos. 24, 36, 45 and 57 of Table 3 orwhich was just below the value at which melting would occur during thesintering. Therefore, the amount of Ca to be added to magnetite wasappropriately in the range of 1.43 to 64.0% by weight.

It has been found from the above experimental results that powder(magnetic oxide powder) having an arbitrary saturation magnetization inwhich magnetite and nonmagnetic phase are coexistent can be formed bysintering at 550° to 1450° C. for 2 hr a mixed powder obtained by mixing1.43 to 64.0% by weight of Ca with magnetite.

FIG. 2 illustrates an exemplary heating curve suitable for the presentinvention. The curve shows an exemplary course of heating in whichheating is conducted at T°C. for 2 hr. In particular, the heating is soperformed that the temperature is elevated at a rate of 200° C./hr fromroom temperature to T°C., which is held for 2 hr. Thereafter, thetemperature is lowered to room temperature at a rate of 200° C./hr.Herein, T°C. is the heating temperature specified in Tables 1 to 3.

FIG. 3 illustrates the saturation magnetization referred to in thepresent invention. In particular, it is an explanatory view of themeasurement of the saturation magnetization shown in Table 1 to 3. Theaxis of abscissa indicates the intensity of applied magnetic field (HOe) while the axis of ordinate indicates the corresponding intensity ofmagnetization (M emu). The intensity of magnetization (Ms emu) of powderin which magnetite and nonmagnetic phase are coexistent is measured bythe use of an vibrating sample magnetometer while keeping theapplication of, for example, 15 kOe magnetic field as specified in FIG.3. The saturation magnetization is calculated by the following formulaspecified in the figure:

    δ=Ms/[weight (g) of sample powder][emu/g]

The values calculated by the above formula are shown as the saturationmagnetization δ s in Tables 1 to 3.

In the above embodiment, the saturation magnetization of magnetite isregulated only through the nonmagnetic phase created by the addition ofCa. In this connection, the saturation magnetization may further finelybe regulated by the formation of an oxide phase (hematite, calciumperoxide, etc.), for example, on the surface of the magnetite powder.

Second Embodiment

The production of magnetic oxides having desired saturationmagnetization values by mixing Ti compounds with magnetite ormagnetite+hematite will now be described referring to FIGS. 4 and 5 andTables 5 to 7.

    ______________________________________                                        magnetite     47.3-98.8% by                                                                             9.8-98.8% by                                                      weight      weight                                              hematite      none        0.0-79.0% by                                                                  weight                                              Ti compound   1.2-52.7% by                                                                              1.2-52.7% by                                                      weight      weight                                              ______________________________________                                         (Value for Ti compound: in terms of Ti)                                  

In the mixing step 2, the above mixed powder is mixed with 0.1 to 4.0%by weight of a compound having --C--C or --C═C-- in its molecules(liquid or solid substance). For example, 2% by weight of PVA (polyvinylalcohol) as a binder and 1% by weight of a polycarboxylate as adispersant are added to the mixed powder as a starting material, andfurther water is added for granulation into globular grains. The amountof water added is in the range of 30 to 70% by weight. When the amountof water was less that 30% by weight, the viscosity of a slurry obtainedby milling was too high to be granulated into globular grains. On theother hand, when the amount of water exceeded 70% by weight, the slurryconcentration was too low to be granulated into dense globular grains.

In the wet-milling step 3, the mixture obtained in the mixing step 2 iswet milled in an attrition mill to thereby prepare a slurry having amixed powder concentration of about 50% by weight.

In the granulating step 4, globular grains are formed. In thus step, theslurry is agitated in an attrition mill for 1 hr, and subjected tohot-air drying by a spray dryer to obtain globular grains.

In the sintering step 5, the grains obtained in the granulating step 4are heated at 550° to 1450° C. (with respect to magnetite) in anatmosphere of an inert gas such as nitrogen gas for 2 hr to thereby formmagnetite and another magnetic and/or nonmagnetic phase. The ratio ofthe nonmagnetic phase can be controlled by regulating the amount, interms of Ti, of Ti compound added, so that it is feasible to produce amagnetic oxide having any desired saturation magnetization by changingthe amount of Ti compound added (see FIG. 5). When hematite is presentin part of the magnetite powder, during the 1200° to 1450° C. sinteringstep 5, not only is the thermal transition from hematite to magnetiteeffected in the inert gas (in weakly reducing atmosphere) but also theorganic substance mixed is brought by the heating in the inert gas intothe state of incomplete combustion, in which the hematite is deprived ofoxygen during the thermal decomposition of the organic substance tothereby be reduced so as to markedly promote the conversion tomagnetite.

In the disintegration step 6, the powder in which magnetite andnonmagnetic phase are coexistent, obtained in the sintering step 5, iscrushed into a product.

According to the above steps, magnetite or magnetite+hematite is mixedwith the Ti compound to obtain a mixed powder, and this mixed powder isfurther mixed with a compound having --C--C-- or --C═C-- in itsmolecules (liquid or solid substance) and water, milled well, hot-airdried, granulated into globular grains and sintered in an inert gas at550° to 1450° C. (for only magnetite as the starting material) or 1200°to 1450° C. (for magnetite+hematite), so that the powder in whichmagnetite and another magnetic and/or nonmagnetic phase are coexistent(magnetic oxide powder) can be produced. Thus, it has become feasible tosafely mass-produce the magnetic oxide powder having any desiredsaturation magnetization at a lowered cost.

Table 4 shows sintering experiment results (TiO₂ : 0% by weight). PVA(polyvinyl alcohol) was added as a binder in the amount specified inTable 4 to hematite powder, and 1% by weight of a polycarboxylate as adispersant and further water were added and agitated for granulationinto grains. The resultant grains were sintered at the temperaturespecified in the Table, and the qualitative analysis thereof by powderX-ray diffractometry was conducted. Results are shown in Table 4. Thefollowing has been found from the results.

(1) No single phase magnetite was obtained from only hematite powderwithout the addition of PVA (Sample Nos. 1 to 8), even if the heatingtemperature was changed. The X-ray diffraction pattern shows thepresence of the phase of α-FE₂ O₃ or FeO.

(2) When the amount of PVA added was set at 2% by weight and the heatingtemperature was varied, single phase magnetite (Fe₃ O₄ only) (SampleNos. 13 to 15) was obtained at heating temperatures ranging from 1200°to 1450° C. α-FE₂ O₃ was coexistent at heating temperatures of 1150° C.or below (Sample Nos. 9 to 12) and FeO was coexistent at heatingtemperatures of 1500° C. or above (Sample No. 16). Therefore, it isrequisite that the heating temperature be in the range of 1200° to 1450°C. (Sample Nos. 13 to 15).

(3) At a heating temperature of 1300° C., the amount of PVA added wasvaried between 0.1 and 3.0% by weight. Single phase magnetite wasobtained without exception (Sample Nos. 17 to 22).

It has been found from the above experimental results that single phasemagnetite is formed as long as the amount of PVA added to hematitepowder is 0.1% by weight (4% by weight) and the sintering is conductedat temperatures ranging from 1200° to 1450° C.

Tables 5 and 6 show experimental example results (magnetite plus Ti) ofthe present invention. In this experimental example, 0.0 to 84.4% byweight, in terms of Ti of powdery titanium oxide (TlO₂) was mixed withmagnetite powder, 1.0% by weight of polyvinyl alcohol was added, andfurther water was added and mixed to obtain a slurry having a powderconcentration of 50% by weight. The slurry was agitated in an attritionmill for 1 hr, and granulated by means of a spray dryer. The thusobtained grains were heated at 400° to 1500° C. in nitrogen gas for 2hr. The oxygen concentration of the nitrogen gas was measured by meansof a zirconia oxygen concentration meter. The saturation magnetizationof each heated sample was measured by means of the vibrating samplemagnetometer (see FIG. 3 relating to the first embodiment). The materialphase was identified by the powder X-ray diffractometry. The collapsetest of each sample was conducted by the use of a microcompressiontester (MCTM-500 manufactured by Shimadzu Corp.), and the correspondinggrain strength was calculated according to the following Hiramatsu'sformula.

    (Grain strength)={2.8×(Breaking load of grains)}/{π×(Diameter of grain).sup.2 }

(1) Heating temperature range

As shown in Table 5, the formation of hematite (α-FE₂ O₃) attributed tothe oxidation by trace O contained in nitrogen gas was not detected whenthe heating was performed at 500° C. (Sample Nos. 1 to 12). However, thegrains strength Pa was on the order of E6 (meaning 10⁶), so that thestrength of globular grains formed by joining of a vast plurality ofmagnetite powder particles was weak to a practical disadvantage. On theother hand, it was found that, when the heating was performed at 1500°C. (Sample Nos. 85 to 96), wustite (FeO) was formed, so that the singlephase magnetite (Fe₃ O₄ only) cannot be obtained. Further, it was foundthat, when a Ti compound is mixed with magnetite as a starting materialand the saturation magnetization is arbitrarily regulated by theformation of magnetite and another magnetic and/or nonmagnetic phase,the assurance of a strength required for actual handling by joining ormild sintering is satisfactory because it is not necessary to newlyreduce the concerned magnetite for conversion to magnetite, so thatdesirable saturation magnetization regulation can be made even if theheating is performed at a temperature as low as 550° C.

The above demonstrates that the suitable heating temperature range isfrom 550° to 1450° C.

(2) Amount of Ti added (% by weight)

Upon noting the amount, in terms of Ti, of powdery titanium oxide added(Sample Nos. 13 to 84) within the heating temperature range of 550° to1450° C. set in section (1) above, it was found that the saturationmagnetization was continuously decreased from 90 to 0 emu/g as the aboveamount was increased from 1.2 to 52.7% by weight. When the amount of Tiadded exceeded 52.7% by weight, the saturation magnetization wasdecreased to about 0 emu/g to thereby cause the magnetic oxide to losepracticability.

Therefore, it has been found that the suitable amount, in terms of Ti,of Ti compound added is in the range of 1.2 to 52.7% by weight.

The above demonstrates that desirable single phase magnetite and anothermagnetic and/or nonmagnetic phase can be formed by mixing 1.2 to 52.7%by weight, in terms of Ti, of a Ti compound with magnetite and effectingthe heating at 550° to 1450° C. (provided that 0.2 to 4.0% by weight ofa substance having a carbon-to-carbon single or double bond is alsoadded).

FIG. 5 shows exemplary saturation magnetization curves as functions ofthe amount of Ti added. The saturation magnetization curves are drawn ina graph in which the axis of abscissa indicates the weight percentage,in terms of Ti, of a Ti compound added while the axis of ordinateindicates the corresponding saturation magnetization value. Theyrepresent experimental example results shown in Table 3. In particular,the saturation magnetization curves are line segments formed by plottingdata shown in Table 3 with respect to each of the heating temperaturerange of 550° to 600° C., heating temperature of 800° C. and heatingtemperature range of 1100° to 1450° C. in a graph in which the axis ofabscissa indicates the weight percentage of Ti added while the axis ofordinate indicates the corresponding saturation magnetization valueemu/g and connecting the plots.

Once the saturation magnetization curves of FIG. 5 have beenestablished, therefore, not only can the weight percentage of Ti to beadded for obtaining the desired saturation magnetization emu/g bedetermined but also the saturation magnetization emu/g can contrarily bedetermined with respect to any certain weight percentage of Ti, tothereby ensure production of a magnetic oxide powder having a desiredsaturation magnetization.

Table 6 shows results of another experimental example (1:1 mixture ofmagnetite and hematite, Ti) according to the present invention. It showsexperimental data obtained by varying the amount of Ti added asspecified in the Table while the proportion of magnetite powder tohematite powder was kept at 1:1. In practice, 0.0 to 84.4% by weight, interms of Ti, of titanium oxide (TiO₂) was mixed with a mixture ofmagnetite and hematite powders whose proportion was kept at 1:1, andgranulated in the same manner as specified in Table 5. The resultantgrains were heated at 1100° to 1500° C. in nitrogen gas for 2 hr. Afterthe heating, the samples were measured in the same manner as specifiedin Table 5, and results are shown in Table 6.

(1) Heating temperature range

Table 6 demonstrates that the heating at 1100° C. (Sample Nos. 1 to 12)causes hematite (α-FE₂ O₃) to remain while the heating at 1500° C.(Sample Nos. 37 to 48) forms wustite (FeO), so that the single phasemagnetite (Fe₃ O₄ only) cannot be obtained.

Therefore, it has been found that the suitable heating temperature rangeis from 1200° to 1450° C.

(2) Amount of Ti added (% by weight)

Upon noting the amount, in terms of Ti, of powdery titanium oxide added(Sample Nos. 13 to 36) within the heating temperature range of 1200° to1450° C. set in section (1) above, it was found that the saturationmagnetization was continuously decreased from 87 to 0 eum/g as the aboveamount was increased from 1.2 to 52.7% by weight. When the amount of Tiadded exceeded 52.7% by weight, the saturation magnetization wasdecreased to about 0 emu/g to thereby cause the magnetic oxide to losepracticability.

Therefore, it has been found that the suitable amount, in terms of Ti,of Ti compound to be added is in the range of 1.2 to 52.7% by weight.

The above demonstrates that desirable single phase magnetite and anothermagnetic and/or nonmagnetic phase can be formed by mixing 1.2 to 52.7%by weight, in terms of Ti, of a Ti compound with a 1:1 mixture ofmagnetite and hematite and effecting the heating at 1200° to 1450° C.(provided that 0.1 to 4.0% by weight of substance having a carbon-tocarbon single or double bond is also added).

Table 7 shows results of an additional experimental example (1:3 mixtureof magnetite and hematite, Ti). It shows experimental data obtained byvarying the amount of Ti added as specified in the Table while theproportion of magnetite powder to hematite powder was kept at 1:3. Inpractice, 0.0 to 84.4% by weight, in terms of Ti, of powdery titaniumoxide (TiO₂) was mixed with a mixture of magnetite and hematite powderswhose proportion was kept at 1:3 in the same manner as specified inTable 6, and granulated in the same manner as specified in Table 5. Theresultant grains were heated at 1100° to 1500° C. in nitrogen gas for 2hr. After the heating, the samples were measured in the same manner asspecified in Table 5, and results are shown in Table 7.

(1) Heating temperature range

Table 7 demonstrates that the heating at 1100° C. (Sample Nos. 1 to 12)causes hematite (α-FE₂ O₃) to remain while the heating at 1500° C.(Sample Nos. 37 to 48) forms wustite (FeO), so that the single phasemagnetite (Fe₃ O₄ only) cannot be obtained.

Therefore, it has been found that the suitable heating temperature rangeis from 1200° to 1450° C.

(2) Amount of Ti added (% by weight)

Upon noting the amount, in terms of Ti, of powdery titanium oxide added(Sample Nos. 13 to 36) within the heating temperature range of 1200° to1450° C. set in section (1) above, it was found that the saturationmagnetization was continuously decreased from 87 to 0 emu/g as the aboveamount was increased from 1.2 to 52.7% by weight. When the amount of Tiadded exceeded 52.7% by weight, the saturation magnetization wasdecreased to about 0 emu/g to thereby cause the magnetic oxide to losepracticability.

Therefore, it has been found that the suitable amount, in terms of Ti,of Ti compound to be added is in the range of 1.2 to 52.7% by weight.

The above demonstrates that desirable single phase magnetite and anothermagnetic and/or nonmagnetic phase can be formed by mixing 1.2 to 52.7%by weight, in terms of Ti, of a Ti compound with a 1:3 mixture ofmagnetite and hematite and effecting the heating at 1200° to 1450° C.(provided that 0.1 to 4.0% by weight of a substance having acarbon-to-carbon single or double bond is also added).

Table 8 shows results of a further experimental example (1:4 mixture ofmagnetite and hematite, Ti). It shows experimental data obtained byvarying the amount of Ti added as specified in the Table while theproportion of magnetite powder to hematite powder was kept at 1:4. Inpractice, 0.0 to 84.4% by weight, in terms of Ti, of powdery titaniumoxide (TiO₂) was mixed with a mixture of magnetite and hematite powderswhose proportion was kept at 1:4 in the same manner as specified inTable 6, and granulated in the same manner as specified in Table 5. Theresultant grains were heated at 1100° to 1500° C. in nitrogen gas for 2hr. After the heating, the samples were measured in the same manner asspecified in Table 5, and results are shown in Table 8.

(1) Heating temperature range

Table 8 demonstrates that the heating at 1100° C. (Sample Nos. 1 to 12)causes hematite (α-FE₂ O₃) to remain while the heating at 1500° C.(Sample Nos. 37 to 48) forms wustite (FeO), so that the single phasemagnetite (Fe₃ O₄ only) cannot be obtained.

Therefore, it has been found that the suitable heating temperature rangeis from 1200° to 1450° C.

(2) Amount of Ti added (% by weight)

Upon noting the amount, in terms of Ti, of powdery titanium oxide added(Sample Nos. 13 to 36) within the heating temperature range of 1200° to1450° C. set in section (1) above, it was found that the saturationmagnetization was continuously decreased from 87 to 0 emu/g as the aboveamount was increased from 1.2 to 52.7% by weight. When the amount of Tiadded exceeded 52.7% by weight, the saturation magnetization wasdecreased to about 0 emu/g to thereby cause the magnetic oxide to losepracticability.

Therefore, it has been found that the suitable amount, in terms of Ti,of Ti compound to be added is in the range of 12 to 52.7% by weight

Therefore, it has been found that the suitable amount, in terms of Ti,of Ti compound to be added is in the range of 1.2 to 52.7% by weight.

The above demonstrates that desirable single phase magnetite and anothermagnetic and/or nonmagnetic phase can be formed by mixing 1.2 to 52.7%by weight, in terms of Ti, of a Ti compound with a 1:4 mixture ofmagnetite and hematite and effecting the heating at 1200° to 1450° C.(provided that 0.1 to 4.0% by weight of a substance having acarbon-to-carbon single or double bond is also added).

Detailed descriptions on the heating curves and the saturationmagnetization measurements are omitted in view of the similarity to andobviousness from those for FIGS. 2 and 3 relating to the firstembodiment of the present invention.

Third Embodiment

The production of magnetic oxides having desired saturationmagnetization values by mixing Sn compounds with hematite,hematite+magnetite, or magnetite will now be described referring to FIG.6 and Tables 9 to 11.

Referring now to FIG. 6, hematite, hematite+magnetite or magnetite andan Sn compound are weighed and blended in the following proportions inthe blending step 1.

    ______________________________________                                        hematite           24.0-99.2% by weight                                       hematite + magnetite                                                                             24.0-99.2% by weight, or                                   magnetite          24.0-99.2% by weight                                       together with      0.8-87.6% by weight.                                       Sn compound (in terms of Sn)                                                  ______________________________________                                    

Thus, a mixed powder is obtained.

In the mixing step 2, the above mixed powder is mixed with 0.1 to 4.0%by weight of a compound having --C--C-- or --C═C-- in its molecules(liquid or solid substance). For example, 2% by weight of PVA (polyvinylalcohol) as a binder and 1% by weight of a polycarboxylate as adispersant are added to the mixed powder as a starting material, andfurther water is added for granulation into globular grains. The amountof water added is in the range of 30 to 70% by weight. When the amountof water was less than 30% by weight, the viscosity of a slurry obtainedby milling was too high to be granulated into globular grains. On theother hand, when the amount of water exceeded 70% by weight, the slurryconcentration was too low to be granulated into dense globular grains.

In the wet-milling step 3, the mixture obtained in the mixing step 2 iswet crushed in an attrition mill to thereby prepare a slurry having amixed powder concentration of about 50% by weight.

In the granulating step 4, globular grains are formed. In this step, theslurry is agitated in an attrition mill for 1 hr, and subjected tohot-air drying by a spray dryer to obtain globular grains.

In the sintering step 5, the grains obtained in the granulating step 4are heated at 1200° to 1450° C. (with respect to hematite orhematite+magnetite) or 550° to 1450° C. (with respect to magnetite) inan atmosphere of an inert gas such as nitrogen gas for 2 hr to therebyform magnetite and another magnetic and/or nonmagnetic phase. The ratioof the nonmagnetic phase can be controlled by regulating the amount, interms of Sn, compound added, so that it is feasible to produce amagnetic oxide having any desired saturation magnetization by changingthe amount of Sn compound added. When hematite is present in part of themagnetite powder, not only is the thermal transition from hematite tomagnetite effected in the inert gas (in weakly reducing atmosphere) butalso the organic substance mixed is brought by the heating in the inertgas into the state of incomplete combustion, in which the hematite isdeprived of oxygen during the thermal decomposition of the organicsubstance to thereby be reduced so as to markedly promote the conversionto magnetite.

In the crushing step 6, the powder in which magnetite and anothermagnetic and/or nonmagnetic phase are coexistent, obtained in thesintering step 5, is crushed into a product.

According to the above steps, hematite, hematite+magnetite, or magnetiteis mixed with the Sn compound to obtain a mixed powder, and this mixedpowder is further mixed with a compound having --C--C-- or --C═C-- inits molecules and water, milled well, hot-air dried, granulated intoglobular grains and sintered in an inert gas at 550° to 1450° C. (withrespect to magnetite) to 1200° to 1450° C. (with respect to magnetite)or 1200° to 1450° C. (with respect to hematite or hematite+magnetite),so that the magnetic oxide powder in which magnetite and anothermagnetic and/or nonmagnetic phase are coexistent can be produced. Thus,it has become feasible to safely mass-produce the magnetic oxide powderhaving any desired saturation magnetization at a lowered cost.Descriptions will be made in order below.

Table 9 shows experimental example results (hematite, Sn) according tothis embodiment. In this experimental example, 0.8 to 87.6% by weight,in terms of Sn of powdery tin oxide was mixed with hematite powder, 1.0%by weight of polyvinyl alcohol was added, and further water was addedand mixed to obtain a slurry having a powder concentration of 50% byweight. The slurry was agitated in an attrition mill for 1 hr, andgranulated by means of a spray dryer. The thus obtained grains wereheated at 1100° to 1500° C. in nitrogen gas for 2 hr. The oxygenconcentration of the nitrogen gas was measured by means of the zirconiaoxygen concentration meter. The saturation magnetization of each heatedsample was measured by means of the vibrating sample magnetometer. Thematerial phase was identified by the powder X-ray diffractometry.

(1) Heating temperature range

Table 9 demonstrates that the heating at 1100° C. (Sample Nos. 1 to 10)causes hematite (α-FE₂ O₃) to remain while the heating at 1500° C.(sample Nos. 41 to 50) forms wustite (FeO), so that the single phasemagnetite (Fe₃ O₄ only) cannot be obtained.

Therefore, it has been found that the suitable heating temperature rangeis from 1200° to 1450° C.

(2) Amount of Sn added (% by weight)

Upon noting the amount, in terms of Sn of powdery tin oxide added(Sample Nos. 11 to 40) within the heating temperature range of 1200° to1450° C. set in section (1) above, it was found that the saturationmagnetization was continuously decreased from 91 to 8 emu/g as the aboveamount was increased from 0.8 to 87.6% by weight. When the amount of Snadded exceeded 76.0% by weight, the saturation magnetization becameextremely low to thereby cause the magnetic oxide to losepracticability.

Therefore, it has been found that the suitable amount, in terms of Sn,of Sn compound to be added is in the range of 0.8 to 76.0% by weight.

The above demonstrates that desirable single phase magnetite and anothermagnetic and/or nonmagnetic phase can be formed by mixing 0.8 to 76.0%by weight, in terms of Sn, of an Sn compound with hematite and effectingthe heating at 1200° to 1450° C. (provided that 0.1 to 4.0% by weight ofa substance having a carbon-to-carbon single or double bond is alsoadded). FIG. 7 shows an exemplary saturation magnetization curve as afunction of the amount of Sn added. The saturation magnetization curveis drawn in a graph in which the axis of abscissa indicates the weightpercentage, in terms of Sn, or Sn compound added while the axis ofordinate indicates the corresponding saturation magnetization value. Itrepresents experimental example results shown in Table 9. In particular,the saturation magnetization curve is a line segment formed by plottingdata shown in Table 9 with respect to the heating temperature range of1200° to 1450° C. in a graph in which the axis of abscissa indicates theweight percentage of Sn added while the axis of ordinate indicated thecorresponding saturation magnetization value emu/g and connecting theplots.

Once the saturation magnetization curve of FIG. 7 has been established,therefore, not only can the weight percentage of Sn to be added forobtaining a desired saturation magnetization emu/g be determined butalso the saturation magnetization emu/g can contrarily be determinedwith respect to any given weight percentage of Sn, to thereby ensureproduction of a magnetic oxide powder having a desired saturationmagnetization.

Table 10 shows results of another experimental example (1;1 mixture ofhematite and magnetite, Sn) according to this embodiment. It showsexperimental data obtained by varying the amount of Sn added asspecified in the Table while the proportion of hematite powder tomagnetite powder was kept at 1:1. In practice, 0.0 to 87.6% by weight,in terms of Sn, of powdery tin oxide was mixed with a 1:1 mixture ofhematite and magnetite powders, and granulated in the same manner asspecified in Table 9. The resultant grains were heated at 100° to 1500°C. in nitrogen gas for 2 hr. After the heating, the samples weremeasured in the same manner as specified in Table 9, and results areshown in Table 10.

(1) Heating temperature range

Table 10 demonstrates that the heating at 1100° C. (Sample Nos. 1 to 10)causes hematite (α-FE₂ O₃) to remain while the heating at 1500° C.(Sample Nos. 41 to 50) forms wustite (FeO), so that the single phasemagnetite (Fe₃ O₄ only) cannot be obtained.

Therefore, it has been found that the suitable heating temperature rangeis from 1200° to 1450° C.

(2) Amount of Sn added (% by weight)

Upon noting the amount, in terms of Sn, of powdery tin oxide added(Sample Nos. 11 to 40) within the heating temperature range of 1200° to1450° C. set in section (1) above, it was found that the saturationmagnetization was continuously decreased from 91 to 9 emu/g as the aboveamount was increased form 0.8 to 87.6% by weight. When the amount of Snadded exceeded 76.0% by weight, the saturation magnetization becameextremely small to thereby cause the magnetic oxide to losepracticability.

Therefore, it has been found that the suitable amount, in terms of Sn,of Sn compound to be added is in the range of 0.8 to 76.0% by weight.

The above demonstrates that desirable single phase magnetite and anothermagnetic and/or nonmagnetic phase can be formed by mixing 0.8 to 76.0%by weight, in terms of Sn, of a Sn compound with a 1:1 mixture ofhematite and magnetite and effecting the heating at 1200° to 1450° C.(provided that 0.1 to 4.0% by weight of a substance having acarbon-to-carbon single or double bond is also added).

Table 11 shows results of an additional experimental example (magnetite,Sn) according to this embodiment. In this experimental example, 0.0 to87.6% by weight, in terms of Sn, of powdery tin oxide was mixed withmagnetite powder, and granulated in the same manner as specified inTable 9. The thus obtained grains were heated at 500° to 1500° C. innitrogen gas for 2 hr. After the heating, the samples were measured inthe same manner as specified in Table 9, and results are shown in Table11.

(1) Heating temperature range

Table 11 demonstrates that, during the heating at 500° C. (Sample Nos. 1to 10), the magnetite is oxidized by trace amount of O₂ contained innitrogen gas to form hematite (α-FE₂ O₃) while the heating at 1500° C.(Sample Nos. 81 to 90) forms wustite (FeO), so that the single phasemagnetite (Fe₃ O₄ only) cannot be obtained. When an Sn compound is mixedwith magnetite as a starting material and magnetite and another magneticand/or nonmagnetic phase is formed to attain arbitrary regulation of thesaturation magnetization, there is no need for newly reducing themagnetite to achieve conversion to magnetite, so that it is satisfactoryto obtain the strength required for handling, etc., by bonding or mildsintering. Thus, it has been found that desirable saturationmagnetization regulation can be achieved at a temperature as low as 550°C.

(2) Amount of Sn added (% by weight)

Upon noting the amount, in terms of Sn, of powdery tin oxide added(Sample Nos. 11 to 80) within the heating temperature range of 550° to1450° C. set in section (1) above, it was found that the saturationmagnetization was continuously decreased from 91 to 8 emu/g as the aboveamount was increased from 0.8 to 87.6% by weight. When the amount of Snadded exceeded 76.9% by weight, the saturation magnetization becameextremely low to thereby cause the magnetic oxide to losepracticability.

Therefore, it has been found that the suitable amount, in terms of Sn,of Sn compound to be added is in the range of 0.8 to 76.0% by weight.

The above demonstrates that desirable single phase magnetite and anothermagnetic and/or nonmagnetic phase can be formed by mixing 0.8 to 76.0%by weight, in terms of Sn, of an Sn compound with magnetite andeffecting the heating at 550° to 1450° C. (provided that 0.1 to 4.0% byweight of a substance having a carbon-to-carbon single or double bond isalso added).

As described above, in the second and third embodiments, Ti and Sncompounds are individually mixed with magnetite, magnetite+hematite orhematite, and the resultant mixed powder is mixed with a substancehaving a carbon-to-carbon single or double bond and sintered to therebyproduce a powder comprising magnetite and another magnetic and/ornonmagnetic phase and having an arbitrary saturation magnetizationvalue. Therefore, a magnetic oxide having any desired saturationmagnetization can easily be mass-produced at a lowered cost. Inparticular, a magnetic oxide comprising magnetite and another magneticand/or nonmagnetic phase and having an arbitrary saturationmagnetization value can easily be produced at a lowered cost by a simpleprocess in which a large volume of magnetite, magnetite+hematite orhematite is mixed with a Ti or Sn compound, further mixed with anorganic substance and directly brought into the sintering step 5.

Fourth Embodiment

As shown in FIG. 8, 0.5 to 65% by weight, in terms of Si, of a powderySi compound was weighed and mixed with a member selected from amonghematite, hematite+magnetite and magnetite powders in the blending step1.

In the mixing step 2, the thus mixed powder is mixed with 0.1 to 4.0% byweight of a compound having --C--C-- or --C═C-- in its molecules (liquidor solid). For example, 2% by weight of polyvinyl alcohol as a binderand 1% by weight of a polycarboxylate as a dispersant are added to themixed powder, and further water is added for granulation into globulargrains. The amount of water added is in the range of 30 to 70% byweight. When the amount of water was less that 30% by weight, theviscosity of a slurry obtained by milling was too high to be granulatedinto globular grains. On the other hand, when the amount of waterexceeded 70% by weight, the slurry concentration was too low to begranulated into dense globular grains.

In the wet-milling step 3, the mixture obtained in the mixing step 2 iswet crushed in an attrition mill to thereby prepare a slurry having amixed powder concentration of about 50% by weight.

In the granulating step 4, globular grains are formed. In this step, theslurry is agitated in an attrition mill for 1 hr, and hot-air dried by aspray dryer to obtain globular grains.

In the sintering step 5, the grains obtained in the granulating step 4are heated at 1200° to 1450° C. or 550° to 1450° C. in an atmosphere ofan inert gas (such as nitrogen gas) for 2 hr to thereby form magnetiteand another magnetic and/or nonmagnetic phase. The value of thesaturation magnetization can be controlled by regulating the amount, interms of Si, of Si compound added, so that it is feasible to produce amagnetic oxide having any desired saturation magnetization by changingthe amount of Si compound added (see Tables 13 to 15). When hematite ispresent in part of the hematite or magnetite powder, not only is thethermal transition from hematite to magnetite effected in the inert gas(in weakly reducing atmosphere) but also the organic substance mixed isbrought by the heating in the inert gas into the state of incompletecombustion, in which the hematite is deprived of oxygen during thethermal decomposition of the organic substance to thereby be reduced soas to markedly promote the conversion to magnetite,

In the crushing step 6, the powder in which magnetite and anothermagnetic and/or nonmagnetic phase are coexistent, obtained in thesintering step 5, is crushed into a product.

According to the above steps, 35 to 99.5% by weight of hematite,hematite+magnetite, or magnetite is mixed with 0.6 to 65% by weight, interms of Si, of an Si compound to obtain a mixed powder, and this mixedpowder is further mixed with 0.1 to 4.0% by weight of a compound having--C--C-- or --C═C-- in its molecules and still further water, milledwell, hot-air dried, granulated into globular grains and sintered in aninert gas at 1200° to 1450° C. (with respect to hematite orhematite+magnetite) or 550° to 1450° C. (with respect to magnetiteonly), so that the magnetic oxide powder in which magnetite and anothermagnetic and/or nonmagnetic phase are coexistent can be produced. Thus,it has become feasible to safely mass-produce the magnetic oxide powderhaving any desired saturation magnetization at a lowered cost.Descriptions will be made below.

Table 12 shows sintering experiment results (SiO₂ : 0% by weight)According to this embodiment. In this experiment, PVA (polyvinylalcohol) was added as a binder in an amount specified in the Table tohematite powder, and further 1% by weight of a polycarboxylate as adispersant and water were added and mixed. The mixture was granulated,and sintered at a temperature T specified in the Table. After thesintering, a qualitative analysis by powder X-ray diffractometry wasconducted. Results are shown in the Table. The following has been foundfrom the results.

(1) No single phase magnetite was obtained even with the changed heatingtemperatures from only hematite powder having no PVA added thereto(Sample Nos. 1 to 8). The X-ray diffractometry showed the presence ofthe phase of hematite (α-FE₂ O₃) or wustite (FeO).

(2) When the amount of PVA added was set at 2% by weight and the heatingtemperature was varied, single phase magnetite (Fe₃ O₄ only) wasobtained at heating temperatures ranging from 1200° to 1450° C. (SampleNos. 9 to 12), and wustite (FeO) was coexistent at heating temperaturesof 1500° C. or above (Sample Nos. 16).

Therefore, it is requisite that the heating temperature be in the rangeof 1200° to 1450° C. (Sample Nos. 13 to 15).

(3) At a fixed heating temperature of 1300° C., the amount of PVA addedwas varied between 0.1 and 3.0% by weight. In any case, single phasemagnetite was obtained (Sample Nos. 17 to 22).

From the above experimental results, it has been found that completelysingle phase magnetite can be obtained whenever PVA is added to hematitepowder in an amount of 0.1 to 3% by weight (4% by weight) and wheneverthe sintering performed by heating at 1200° to 1450° C.

Table 13 shows experimental example results (hematite) according to thisembodiment. In this experimental example, 0.0 to 79.5% by weight, interms of Si, of powdery silicon oxide was mixed with hematite powder,and further mixed with water to obtain a slurry having a powderconcentration of 50% by weight. The slurry was agitated in an attritionmill for 1 hr, and dried at 110° C. 1.0% by weight of polyvinyl alcoholwas added to the obtained pellet. The mixture was mixed in a mortar, andsifted through a 425-μm-mesh standard sieve to obtain grains. 0.5 g ofthe resultant grains were put in a cylindrical mold having a diameter of12.5 mm, and shaped under a molding pressure of 1 t/cm². The molding washeated at 1100° to 1500° C. for 2 hr in nitrogen gas. The oxygenconcentration of the nitrogen gas was measured by the use of a zirconiaoxygen concentration meter. The saturation magnetization of each sampleafter the heating was measured by means of a vibrating samplemagnetometer (see FIG. 3 of the first embodiment). Results are shown inTable 13.

(1) Heating temperature range

Table 13 demonstrates that the heating at 1100° C.(Sample Nos. 1 to 10)causes hematite (α-FE₂ O₃) to remain while the heating at 1500° C.(Sample Nos. 41 to 50) forms wustite (FeO), so that the single phasemagnetite (Fe₃ O₄ only) cannot be obtained.

Therefore, it has been found that the suitable heating temperature rangeis from 1200° to 1450° C.

(2) Amount of Si added (% by weight)

Upon noting the amount, in terms of Si, of powdery silicon oxide added(Sample Nos. 11 to 40) within the heating temperature range of 1200° to1450° C. set in section (1) above, it was found that the saturationmagnetization was continuously decreased from 91 to 20 emu/g as theabove amount was increased from 0.5 to 65.0% by weight (see FIG. 9referred to later). When the amount of Si added exceeded as small as 20emu/g or less to thereby cause the magnetic oxide to losepracticability.

Therefore, it has been found that the suitable amount, in terms of Si,of Si compound to be added is in the range of 0.5 to 65% by weight.

The above demonstrates that desirable single phase magnetite+nonmagneticphase can be formed by mixing 0.5 to 65% by weight, in terms of Si, ofan Si compound with hematite and effecting the heating at 1200° to 1450°C. (provided that 0.1 to 4.0% by weight of a substance having acarbon-to-carbon single or double bond is also added).

FIG. 9 shows an exemplary saturation magnetization curve as a functionof the amount of Si added in this embodiment. The saturationmagnetization curve is drawn in a graph in which the axis of abscissaindicates the weight percentage, in terms of Si, of Si compound addedwhile the axis of ordinate indicates the corresponding saturationmagnetization value. It represents experimental example results shown inTable 13. In particular, the saturation magnetization curve is a linesegment formed by plotting data shown in Table 18 with respect to theheating temperature range of 1200° to 1450° C. in a graph in which theaxis of abscissa indicates the weight percentage of Si added while theaxis of ordinate indicates the corresponding saturation magnetizationvalue emu/g and connecting the plots.

Once the saturation magnetization curve of FIG. 9 has been established,therefore, not only can the weight percentage of Si to be added forobtaining a desired saturation magnetization emu/g be determined butalso the saturation magnetization emu/g can contrarily be determinedwith respect to any given weight percentage of Si, to thereby ensureproduction of a magnetic oxide powder having a desired saturationmagnetization.

Table 14 shows results of another experimental example(hematite+magnetite) according to this embodiment. In the experimentalexample, 0.0 to 79.5% by weight, in terms of Si, of powdery siliconoxide was mixed with a 50% by weight: 50% by weight mixture of hematiteand magnetite powders (hematite+magnetite), and the resultant mixedpowder was mixed with water, milled, granulated, molded and heated inthe same manner as specified in Table 13 to thereby obtain samples. Thesamples were measured, and results are shown in Table 14.

(1) Heating temperature range

Table 14 demonstrates that the heating at 1100° C. (Sample Nos. 1 to 10)causes hematite (α-FE₂ O₃) to remain while the heating at 1500° C.(Sample Nos. 41 to 50) forms wustite (FeO), so that the single phasemagnetite (Fe₃ O₄ only) cannot be obtained.

Therefore, it has been found that the suitable heating temperature rangeis from 1200° to 1450° C.

(2) Amount of Si added (% by weight)

Upon noting the amount, in terms of Si, of powdery silicon oxide added(Sample Nos. 11 to 40) within the heating temperature range of 1200° to1450° C. set in section (1) above, it was found that the saturationmagnetization was continuously decreased from 91 to 20 emu/g as theabove amount was increased from 0.5 to 65.0% by weight. When the amountof Si added exceeded 65% by weight, the saturation magnetization becameas small as 20 emu/g to thereby cause the magnetic oxide to losepracticability.

Therefore, it has been found that the suitable amount, in terms of Si,of Si compound to be added is in the range of 0.5 to 65% by weight.

The above demonstrates that desirable single phase magnetite and anothermagnetic and/or nonmagnetic phase can be formed by mixing 0.5 to 65% byweight, in terms of Si, of a Si compound with hematite+magnetite andeffecting the heating at 1200° to 1450° C. (provided that 0.1 to 4.0% byweight of a substance having a carbon-to-carbon single or double bond isalso added).

Table 15 shows results of a further experimental example (magnetite)according to this embodiment. In this experimental example, 0.0 to 79.5%by weight, in terms of Si, of powdery silicon oxide was mixed withmagnetite powder and further mixed with water in the same manner asspecified in Table 13 to obtain a slurry having a powder concentrationof 50% by weight. The slurry was agitated in an attrition mill for 1 hr,and dried at 110° C. 1.0% by weight of polyvinyl alcohol was added tothe obtained pellet. The mixture was mixed in a mortar, and siftedthrough a 425-μm-mesh standard sieve to obtain grains. 0.5 g of theresultant grains were put in a cylindrical mold having a diameter of12.5 mm, and shaped under a molding pressure of 1 t/cm². The molding washeated at 500° to 1500° C. for 2 hr in nitrogen gas. The oxygenconcentration of the nitrogen gas was measured by the use of a zirconiaoxygen concentration meter. The saturation magnetization of each sampleafter the heating was measured by means of a vibrating samplemagnetometer (see FIG. 3 of the first embodiment). Results are shown inTable 15.

(1) Heating temperature range

Table 15 demonstrates that, during the heating at 500° C. (Sample Nos. 1to 10), the magnetite is oxidized by trace O₂ contained in nitrogen gasto form hematite (α-FE₂ O₃) while the heating at 1500° C. (Sample Nos.81 to 90) forms wustite (FeO), so that the single phase magnetite (Fe₃O₄ only) cannot be obtained. When an Si compound is mixed with magnetiteas a starting material and magnetite and another magnetic and/ornonmagnetic phase is formed to attain arbitrary regulation of thesaturation magnetization, there is no need for newly reducing themagnetite to achieve conversion to magnetite, so that it is satisfactoryto obtain the strength required for handling and other practicaloperation by bonding or mild sintering. Thus, it has been found thatdesirable saturation magnetization regulation can be achieved at atemperature as low as 550° C.

Therefore, it has been found that the suitable heating temperature rangeis from 550° to 1450° C.

(2) Amount of Si added (% by weight)

Upon noting the amount, in terms of Si, of powdery silicon oxide added(Sample No. 11 to 80) within the heating temperature range of 550° to1450° C. set in section (1) above, it was found that the saturationmagnetization was continuously decreased from 91 to 16 emu/g as theabove amount was increased from 0.5 to 65.0% by weight. When the amountof Si added exceeded 65% by weight, the saturation magnetization becameas small as 16 emu/g or less to thereby cause the magnetic oxide to losepracticability.

Therefore, it has been found that the suitable amount, terms of Si, ofSi compound to be added is in the range of 0.5 to 65% by weight.

The above demonstrates that desirable single phase magnetite and anothermagnetic and/or nonmagnetic phase can be formed by mixing 0.5 to 65% byweight, in terms of Si, of an Si compound with magnetite and effectingthe heating at 550° to 1450° C. (provided that 0.1 to 4.0% by weight ofa substance having a carbon-to-carbon single or double bond is alsoadded).

Detailed descriptions on the heating curbed for 2-hr heating at theheating temperature T°C. (heating temperature specified in Tables 12 to15) and the saturation magnetization relating to the above embodimentare omitted in view of the similarity to and obviousness from those formFIGS. 2 and 3 relating to the first embodiment of the present invention.

Besides the above embodiment, it has been found that the saturationmagnetization may further finely be regulated by controlling thesintering conditions so as to form an oxide phase (hematite calciumperoxide, etc.), for example, on the surface of the powder. This isapplicable according to necessity although additional operation isrequired.

As described above, in the forgoing embodiment, an Si compound and asubstance having a carbon-to-carbon single or double bond are mixed withhematite, hematite+magnetite, or magnetite, and sintered to therebyproduce a powder comprising magnetite and another magnetic and/ornonmagnetic phase and having an arbitrary saturation magnetizationvalue. Therefore, a magnetic oxide having any desired saturationmagnetization can easily be mass-produced at a lowered cost. Inparticular, a powder (magnetic oxide) comprising magnetite and anothermagnetic and/or nonmagnetic phase and having an arbitrary saturationmagnetization value can easily be produced at a lowered cost by a simpleprocess in which a mixed powder comprising a large volume of hematite,hematite+magnetite, or magnetite, an Si compound and a substance havinga carbon-to-carbon single or double bond is directly brought into thesintering step 5.

Fifth Embodiment

Referring to FIG. 10, hematite or hematite+magnetite is blended with0.20 to 18.50% by weight of Mg or alternatively magnetite is blendedwith 0.20 to 26.20% by weight of Mg, optionally together with 0.20 to10.00% by weight of Mn (the upper limit of the total of the former twois 18.50% by weight while that of the latter is 26.20% by weight) tothereby form a mixed powder in the blending step 1. The term "Mg" meansthe Mg moiety, expressed in % by weight, of organic and inorganiccompounds each containing Mg, such as MgCO₃, Mg(OH)₂, MgCl₂ nH₂ O andMgO. Likewise, the term "Mn" means the Mn moiety expressed in % byweight of organic and inorganic compounds each containing Mn, such asMnO₂, Mn₃ O₄, MnOOH, MnCO₃, Mn₂ O₃ and MnO. The terms "Mg" and "Mn" areemployed for brevity in this specification. 0.20 to 18.50% by weight or0.20 to 26.20% by weight of Mg optionally together with 0.20 to 10.00%by weight of Mn is blended with the balance (q.s. ad 100%) of hematite,hematite+magnetite, or magnetite (that is, for example, the weightpercentage of the "O" moiety of MgO is not contained in the value of100% by weight). The terminology "hematite+magnetite" employed hereinmeans a mixture (mixed powder) of hematite and magnetite. In particular,the magnetite as a starting material may be either of:

(1) magnetite powder (produced in the house or purchased from othercompanies), and

(2) magnetite powder obtained by pulverizing a product having anoff-specification particle size (recovered item) so as for the same tohave the given particle size.

The product grains described later are those each composed of, forexample, 103 to 107 starting powder particles (1-3 μm) gathered into aglobular form having a diameter of 50 to 100 μm. Thus, any once producedmagnetite powder product (off-specification item) can be pulverized tothereby easily obtain the desired starting material powder.

In the mixing step 2, the above mixed powder is mixed with 0.1 to 4.0%by weight of a compound having --C--C-- or --C═C-- in its molecules(liquid or solid). For example, 2% by weight of polyvinyl alcohol as abinder and 1% by weight of a polycarboxylate as a dispersant are addedto the mixed powder, and further water is added for granulation intoglobular grains. The amount of water added is in the range of 30 to 70%by weight. When the amount of water was less than 30% by weight, theviscosity of the slurry obtained by milling was too high to begranulated into globular grains. On the other hand, when the amount ofwater exceeded 70% by weight, the slurry concentration was too low to begranulated into dense globular grains.

In the wet-milling step 3, the mixture obtained in the mixing step 2 iswet crushed in an attrition mill to thereby prepare a slurry having amixed powder concentration of about 50% by weight.

In the granulating step 4, the powder is formed into globular grains. Inthis step, the slurry is agitated in an attrition mill (attritor) for 1hr, and hot-air dried by a spray dryer to obtain globular grains.

In the sintering step 5, the grains obtained in the granulating step 4are heated at 550° to 1500° C. in an atmosphere of an inert gas (such asnitrogen gas) for 2 hr to thereby obtain powder in which single phasemagnetite and nonmagnetic phase are coexistent. The value of saturationmagnetization of the powder can be regulated by the amount of Mg added,so that it is feasible to produce a powdery magnetic oxide having anydesired saturation magnetization by changing the amount of Mg added (seeTables 16 to 18). When hematite is present in part of the hematite ormagnetite powder, during the 550° to 1500° C. sintering step 5, not onlyis the thermal transition from hematite to magnetite effected in theinert gas (in weakly reducing atmosphere) but also the organic substancemixed is brought by the heating in the inert gas into the stage ofincomplete combustion, in which the hematite is deprived of oxygenduring the thermal decomposition of the organic substance to thereby bereduced to markedly promote the conversion to magnetite. The mixing ofMn improves the grain strength of the powder (see Table 10 and FIG. 12).

In the disintegration step 6, the powder in which magnetite and anothermagnetic and/or nonmagnetic phase are coexistent, obtained in thesintering step 5, is disintegrated into a product.

According to the above steps, hematite, hematite+magnetite, or magnetiteis mixed with the above Mg, optionally together with Mn for improvingthe grain strength of the powder, to obtain a mixed powder, and thismixed powder is further mixed with a compound having --C--C-- or --C═C--in its molecules and water, milled well, hot-air dried, granulated intoglobular grains and sintered in an inert gas at 550° to 1500° C., sothat the magnetic oxide powder in which magnetite and nonmagnetic phaseare coexistent can be produced. Thus, it has become feasible to safelymass-produce a magnetic oxide powder having any desired saturationmagnetization and a satisfactory grain strength at a lowered cost.Descriptions will be made below.

Table 16 shows sintering experiment results (hematite+Mg) according tothis embodiment. In the experiment, 0.00 to 40.00% by weight, in termsof Mg, of powder of magnesium oxide (MgO) was blended with hematitepowder, and mixed with water to obtain a slurry having a powderconcentration of 50% by weight. 1.0% by weight of polyvinyl alcohol wasadded to the slurry, and agitated in an attrition mill for 1 hr. Theslurry was spray dried by the use of a spray dryer to obtain grains. Thethus obtained grains were heated at 500° to 1500° C. in nitrogen gas for2 hr. After the heating, the saturation magnetization of each sample wasmeasured by means of a vibrating sample magnetometer. The grain strengthof each sample was determined by measuring with the use of amicro-compression tester (MCTM-500 manufactured by Shimadzu Corp.) andcalculating according to the following Hiramatsu's formula.

    (Grain strength)={2.8×(Breaking load of grain)}/{ρ×(Diameter of grain).sup.2 }

(1) When the heating temperature was 500° C., the grain strength was assmall as less than 1E7 Pa to thereby cause the magnetic oxide to fall toserve practical use. Thus, 500° C. is unacceptable. On the other hand,when the heating temperature was 550° C., the grain strength was notsmaller than 1E7 Pa, so that the magnetic oxide would serve practicaluse. Therefore, it has been determined that 550° C. is an appropriatelower limit of the heating temperature.

(2) When the heating temperature was 1500° C., the conversion tomagnetite was so incomplete as to form wustite (FeO) with the resultthat the saturation magnetization began to become as small as 86 emu/gor less as noted with respect to Sample No. 91. Although satisfactorysaturation magnetization can be obtained at higher temperatures, it hasbeen determined that 1500° C. is an appropriate upper limit of theheating temperature from the viewpoint of the practical heating limit ofelectric furnaces.

(3) When the amount of Mg added was 0.20% by weight (Sample Nos. 13, 23,33, 43, 53, 63, 73, 83 and 93) at which the saturation magnetizationbegan to decrease within the heating temperature range of 550° to 1500°C., a saturation magnetization of 90 to 82 emu/g was obtained at 550° to1100° C.

Thus, it has been determined that 0.20% by weight is an appropriatelower limit of the amount of Mg added. On the other hand, the amount ofMg added at which the saturation magnetization was at least 10 emu/g was18.50% by weight (Sample Nos. 18, 28, 38, 48, 58, 68, 78, 88 and 98) at550° to 1500° C. Thus, it has been determined that 18.50% by weight isan appropriate upper limit of the amount of Mg added.

It has been found from the above experimental results that a powder(magnetic oxide) comprising magnetite and another magnetic and/ornonmagnetic phase admixed together and having an arbitrary saturationmagnetization as indicated by the hatched portion in FIG. 11(a)described later can be formed by mixing 0.20 to 18.50% by weight of Mgwith hematite and sintering the resultant mixed powder at 550° to 1500°C. for 2 hr.

Table 17 shows results of another sintering experiment(hematite+magnetite+Mg) according to this embodiment.

In the experiment, 0.00 to 40.0% by weight, in terms of of powder ofmagnesium oxide (MgO) was blended with a 1:1 mixture of hematite andmagnetite powders, treated in the same manner as specified in Table 16,and measured.

(1) When the heating temperature was 500° C., the grain strength was assmall as less that 1E7 Pa to thereby cause the magnetic oxide to fall toserve practical use. Thus, 500° C. is unacceptable. On the other hand,when the heating temperature was 550° C., the grain strength was notsmaller than 1E7 Pa, so that the magnetic oxide would serve practicaluse. Therefore, it has been determined that 550° C. is an appropriatelower limit of the heating temperature.

(2) When the heating temperature was 1500° C., the conversion tomagnetite was so incomplete as to form wustite (FeO) with the resultthat the saturation magnetization began to become as small as 86 emu/gor less as noted with respect to Sample No. 91. Although satisfactorysaturation magnetization can be obtained at higher temperatures, it hasbeen determined that 1500° C. is an appropriate upper limit of theheating temperature from the viewpoint of the practical heating limit ofelectric furnaces.

(3) When the amount of Mg added was 0.20% by weight (Sample Nos. 13, 23,33, 43, 53, 63, 73, 83 and 93) at which the saturation magnetizationbegan to decrease within the heating temperature range of 550° to 1500°C., a saturation magnetization of 61 emu/g was obtained at 550° to 1100°C. while a saturation magnetization of 90 to 82 emu/g was obtained at1200° to 1500° C. Thus, it has been determined that 0.20% by weight isan appropriate power limit of the amount of Mg added. On the other hand,the amount of Mg added at which the saturation magnetization was atleast 10 emu/g was 18.50% by weight (Sample Nos. 18, 28, 38, 48, 58, 68,78, 88 and 98) at 550° to 1500° C. Thus, it has been determined that18.50% by weight is an appropriate upper limit of the amount of Mgadded.

It has been found from the above experimental results that a powder(magnetic oxide) comprising magnetite and another magnetic and/ornonmagnetic phase admixed together and having an arbitrary saturationmagnetization as indicated by the hatched portion in FIG. 11(b)described later can be formed by mixing 0.20 to 18.50% by weight of Mgwith hematite+magnetite and sintering the resultant mixed powder at 550°to 1500° C. for 2 hr.

Table 18 shows results of a further sintering experiment (magnetite+Mg)according to this embodiment. In the experiment, 0.00 to 40.0% byweight, in terms of Mg, of powder of magnesium oxide (MgO) was blendedwith magnetite, treated in the same manner as specified in Table 16, andmeasured.

(1) When the heating temperature was 500° C., the grain strength was assmall as less than 1E7 Pa to thereby cause the magnetic oxide to fail toserve practical use. Thus, 500° C. is unacceptable. On the other hand,when the heating temperature was 550° C., the grain strength was notsmaller than 1E7 Pa, so that the magnetic oxide would serve practicaluse. Therefore, it has been determined that 550° C. is an appropriatelower limit of the heating temperature.

(2) When the heating temperature was 1500° C., the conversion tomagnetite was so incomplete as to form wustite (FeO) with the resultthat the saturation magnetization began to become as small as 86 emu/gor less as noted with respect to sample No. 91. Although satisfactorysaturation magnetization can be obtained at higher temperatures, it hasbeen determined that 1500° C. is an appropriate upper limit of theheating temperature from the viewpoint of the practical heating limit ofelectric furnaces.

(3) When the amount of MK added was 0.20% by weight (Sample Nos. 13, 23,33, 43, 53, 63, 73, 83 and 93) at which the saturation magnetizationbegan to decrease within the heating temperature range of 550° to 1500°C., a saturation magnetization of 91 to 82 emu/g was obtained at 550° to1500° C. Thus, it has been determined that 0.20% by weight is anappropriate lower limit of the amount of Mg added. On the other hand,the amount of Mg added at which the saturation magnetization was at mostabout 10 emu/g was 26.20% by weight (Sample Nos. 19, 29, 39 and 49) at550° to 1100° C. and 19.50% by weight (Sample Nos. 58, 68, 78, 88 and98) at 1200° to 1500° C. It has been determined, taking the above twovalues into account, that 26.20% by weight is an appropriate upper limitof the amount of Mg added.

It has been found from the above experimental results that a powder(magnetic oxide) comprising magnetite and another magnetic and/ornonmagnetic phase admixed together and having an arbitrary saturationmagnetization as indicated by the hatched portion in FIG. 11(a)described later can be formed by mixing 0.20 to 26.20% by weight of Mgwith magnetite and sintering the resultant mixed powder at 550° to 1100°C. for 2 hr.

FIGS. 11(a), 11(b) and 11(c) are explanatory vies of the scope ofvariation of saturation magnetization by regulation. These are summariesof the above-mentioned experimental results of Tables 16 to 18, made foreasy understanding. The axis of abscissa indicates the heatingtemperature, the axis of ordinate the saturation magnetization byregulation.

FIG. 11(a) shows the scope of variation of saturation magnetization byregulation, based on Table 16 relating to hematite+Mg.

(1) When the heating temperature is in the range of 550° to 1100° C.,the magnetic oxide can have an arbitrary saturation magnetizationranging from 40 to 10 emu/g as indicated by the hatched portion in thefigure by changing the amount of Mg added within the range of 0.20 to18.50% by weight, as described with respect to Table 16.

(2) When the heating temperature is in the range of 1200° to 1500° C.,the magnetic oxide can have an arbitrary saturation magnetizationranging from 90 to 10 emu/g as indicated by the hatched portion in thefigure by changing the amount of Mg added within the range of 0.20 to18.50% by weight, as described with respect to Table 16.

FIG. 11(b) shows the scope of variation of saturation magnetization byregulation, based on Table 17 relating to hematite+magnetite+Mg.

(1) When the heating temperature is in the range of 550° to 1100° C.,the magnetic oxide can have an arbitrary saturation magnetizationranging from 60 to 10 emu/g as indicated by the hatched portion in theFigure by changing the amount of Mg added within the range of 0.20 to18.50% by weight, as described with respect to Table 17.

FIG. 11(c) shows the scope of variation of saturation magnetization byregulation, based on Table 18 relating to magnetite+Mg.

(1) When the heating temperature is in the range of 550° to 1100° C.,the magnetic oxide can have an arbitrary saturation magnetizationranging from 90 to 10 emu/g as indicated by the hatched portion in theFigure by changing the amount of Mg added within the range of 0.20 to26.20% by weight, as described with respect to Table 18.

Table 19 shows results of a still further sintering experiment(hematite+Mg+Mn) according to this embodiment. In the experiment, 3.60%by weight, in terms of of powder of magnesium oxide (MgO) was blendedwith hematite powder, and further 0.00 to 10.00% by weight, in terms ofMn, of manganese oxide (Mn₃ O₄)was added thereto. The resultant mixturewas treated under the same conditions as specified in Table 16.

(1) When the heating temperature was 550° C., the grain strength wasgradually increased from 13E8 Pa of Sample No. 16 of Table 16 notcontaining Mn to 17E8 Pa in accordance with the increase of the amountof Mn added as shown in Table 19, thus demonstrating the contribution ofthe addition of Mn to the grain strength. The saturation magnetizationwas also increased from 27 emu/g to 39 emu/g in accordance with theincrease of the amount of Mn added. The increase of the saturationmagnetization is caused by the incorporation of Mn which itself ismagnetized.

(2) When the heating temperature was 1300° C., the grain strength wasgradually increased from 36E9 Pa of Sample No. 66 of Table 16 notcontaining Mn to 52E9 Pa in accordance with the increase of the amountof Mn added as shown in Table 19, thus demonstrating the contribution ofthe addition of M to the grain strength. The saturation magnetizationwas also increased from 67 emu/g to 80 emu/g in accordance with theincrease of the amount of Mn added. The increase of the saturationmagnetization is caused by the incorporation of Mn which itself ismagnetized.

FIGS. 12(a) and 12(b) are explanatory views of the effect of theaddition of Mn on the grain strength of the Mg blend according to thisembodiment. These are graphs of the results of sintering experimentshown in Table 19, prepared for easy understanding. The axis of abscissaindicates the amount of Mn added, % by weight, while the axis ofordinate indicates the grain strength Pa.

FIG. 12 (a) shows the relationship between the amount of Mn added andthe grain strength with respect to the magnetic oxide produced bysintering a powder obtained by mixing hematite with 3.60% by weight ofMg and further with Mn at 550° C. in N₂ for 2 hr as specified in Table19. This graph demonstrates the effect of the addition of Mn inincreasing the grain strength of the sintered magnetic oxide.

FIG. 12(b) shows the relationship between the amount of Mn added and thegrain strength with respect to the magnetic oxide produced by sinteringa powder obtained by mixing hematite with 3.60% by weight of Mg andfurther with Mn at 1300° C. in N₂ for 2 hr as specified in Table 19.This graph demonstrates the effect of the addition of Mn in increasingthe grain strength of the sintered magnetic oxide.

FIG. 13 shows an exemplary heating temperature curve according to thisembodiment. The curve is employed in the experiments relating to theabove-mentioned Table 16 and FIGS. 12(a) and 12(b). In the sintering,the heating is conducted in an atmosphere of N₂ at a rate of 200° C./hrto a given temperature, which is held for 2 hr. Thereafter, the coolingis conducted at a rate of 200° C./hr.

As described above, in the foregoing embodiment, an Si compound and asubstance having a carbon-to-carbon single or double bond are mixed withhematite, hematite+magnetite, or magnetite, and sintered to therebyproduce a powder comprising magnetite and nonmagnetic phase admixedtogether and having an arbitrary saturation magnetization value.Therefore, a magnetic oxide having any desired saturation magnetizationcan easily be mass-produced at a lowered cost. In particular, a powder(magnetic oxide) comprising magnetite and another magnetic and/ornonmagnetic phase admixed together and having an arbitrary saturationmagnetization value can easily be produced at a lowered cost by a simpleprocess in which a mixed powder comprising a large volume of hematite,hematite+magnetite, or magnetite, an Si compound and a substance havinga carbon-to-carbon single or double bond is directly brought into thesintering step 5.

                                      TABLE 1                                     __________________________________________________________________________    Firing Experiment Results (Hematite) of the Present Invention                     Amt. of Ca                                                                          Heating   Satn.                                                     Sample                                                                            added temp.                                                                              Atmo-                                                                              magnetization                                                                        Results of powder X-ray diffractometry             No. (wt %)                                                                              T (°C.)                                                                     sphere                                                                             δs (emu/g)                                                                     α-Fe.sub.2 O.sub.3                                                             Fe.sub.3 O.sub.4                                                                     FeO                                  __________________________________________________________________________    1   0.00  1100 N.sub.2                                                                            80     found  found  none                                 2   0.71  "    "    82     found  found  none                                 3   1.43  "    "    80     found  found  none                                 4   3.55  "    "    75     found  found  none                                 5   4.97  "    "    72     found  found  none                                 6   7.10  "    "    68     found  found  none                                 7   10.65 "    "    64     found  found  none                                 8   14.30 "    "    60     found  found  none                                 9   0.00  1200 N.sub.2                                                                            92     none   found  none                                 10  0.71  "    "    88     none   found  none                                 11  1.43  "    "    88     none   found  none                                 12  3.55  "    "    86     none   found  none                                 13  4.97  "    "    80     none   found  none                                 14  7.10  "    "    77     none   found  none                                 15  10.65 "    "    molten immeasurable                                                                         immeasurable                                                                         immeasurable                         16  14.30 "    "    "      "      "      "                                    17  0.00  1300 N.sub.2                                                                            92     none   found  none                                 18  0.71  "    "    88     none   found  none                                 19  1.43  "    "    87     none   found  none                                 20  3.55  "    "    86     none   found  none                                 21  4.97  "    "    81     none   found  none                                 22  7.10  "    "    77     none   found  none                                 23  10.65 "    "    molten immeasurable                                                                         immeasurable                                                                         immeasurable                         24  14.30 "    "    "      "      "      "                                    25  0.00  1450 N.sub.2                                                                            92     none   found  none                                 26  0.71  "    "    88     none   found  none                                 27  1.43  "    "    87     none   found  none                                 28  3.55  "    "    85     none   found  none                                 29  4.97  "    "    80     none   found  none                                 30  7.10  "    "    76     none   found  none                                 31  10.65 "    "    molten immeasurable                                                                         immeasurable                                                                         immeasurable                         32  14.30 "    "    "      "      "      "                                    33  0.00  1500 N.sub.2                                                                            85     none   found  found                                34  0.71  "    "    85     none   found  found                                35  1.43  "    "    87     none   found  found                                36  3.55  "    "    80     none   found  found                                37  4.97  "    "    77     none   found  found                                38  7.10  "    "    molten none   found  found                                39  10.65 "    "    "      immeasurable                                                                         immeasurable                                                                         immeasurable                         40  14.30 "    "    "      "      "      "                                    __________________________________________________________________________

                                      TABLE 2                                     __________________________________________________________________________    Firing Experiment Results (Hematite + Magnetite) of the Present               Invention                                                                         Amt. of Ca                                                                          Heating   Satn.                                                     Sample                                                                            added temp.                                                                              Atmo-                                                                              magnetization                                                                        Results of powder X-ray diffractometry             No. (wt %)                                                                              T (°C.)                                                                     sphere                                                                             δs (emu/g)                                                                     α-Fe.sub.2 O.sub.3                                                             Fe.sub.3 O.sub.4                                                                     FeO                                  __________________________________________________________________________    1   0.00  1100 N.sub.2                                                                            67     found  found  none                                 2   0.71  "    "    66     found  found  none                                 3   1.43  "    "    65     found  found  none                                 4   3.55  "    "    64     found  found  none                                 5   4.97  "    "    61     found  found  none                                 6   7.10  "    "    56     found  found  none                                 7   10.65 "    "    51     found  found  none                                 8   14.30 "    "    47     found  found  none                                 9   0.00  1200 N.sub.2                                                                            92     none   found  none                                 10  0.71  "    "    89     none   found  none                                 11  1.43  "    "    88     none   found  none                                 12  3.55  "    "    86     none   found  none                                 13  4.97  "    "    80     none   found  none                                 14  7.10  "    "    77     none   found  none                                 15  10.65 "    "    molten immeasurable                                                                         immeasurable                                                                         immeasurable                         16  14.30 "    "    "      "      "      "                                    17  0.00  1300 N.sub.2                                                                            92     none   found  none                                 18  0.71  "    "    88     none   found  none                                 19  1.43  "    "    88     none   found  none                                 20  3.55  "    "    86     none   found  none                                 21  4.97  "    "    81     none   found  none                                 22  7.10  "    "    75     none   found  none                                 23  10.65 "    "    molten immeasurable                                                                         immeasurable                                                                         immeasurable                         24  14.30 "    "    "      "      "      "                                    25  0.00  1450 N.sub.2                                                                            92     none   found  none                                 26  0.71  "    "    88     none   found  none                                 27  1.43  "    "    88     none   found  none                                 28  3.55  "    "    85     none   found  none                                 29  4.97  "    "    82     none   found  none                                 30  7.10  "    "    76     none   found  none                                 31  10.65 "    "    molten immeasurable                                                                         immeasurable                                                                         immeasurable                         32  14.30 "    "    "      "      "      "                                    33  0.00  1500 N.sub.2                                                                            87     none   found  found                                34  0.71  "    "    85     none   found  found                                35  1.43  "    "    86     none   found  found                                36  3.55  "    "    80     none   found  found                                37  4.97  "    "    77     none   found  found                                38  7.10  "    "    molten none   found  found                                39  10.65 "    "    "      immeasurable                                                                         immeasurable                                                                         immeasurable                         40  14.30 "    "    "      "      "      "                                    __________________________________________________________________________

                                      TABLE 3                                     __________________________________________________________________________    Firing Experiment Results (Magnetite) of the Present Invention                    Amt. of Ca                                                                          Heating   Satn.                                                     Sample                                                                            added temp.                                                                              Atmo-                                                                              magnetization                                                                        Results of powder X-ray diffractometry             No. (wt %)                                                                              T (°C.)                                                                     sphere                                                                             δs (emu/g)                                                                     α-Fe.sub.2 O.sub.3                                                             Fe.sub.3 O.sub.4                                                                     FeO                                  __________________________________________________________________________    1   0.00  500  N.sub.2                                                                            86     found  found  none                                 2   0.71  "    "    85     found  found  none                                 3   1.43  "    "    84     found  found  none                                 4   3.55  "    "    83     found  found  none                                 5   4.97  "    "    80     found  found  none                                 6   7.10  "    "    77     found  found  none                                 7   10.65 "    "    72     found  found  none                                 8   14.30 "    "    69     found  found  none                                 9   21.30 "    "    60     found  found  none                                 10  35.50 "    "    44     found  found  none                                 11  56.80 "    "    18     found  found  none                                 12  64.00 "    "     9     found  found  none                                 13  0.00  550  N.sub.2                                                                            91     none   found  none                                 14  0.71  "    "    90     none   found  none                                 15  1.43  "    "    90     none   found  none                                 16  3.55  "    "    87     none   found  none                                 17  4.97  "    "    85     none   found  none                                 18  7.10  "    "    82     none   found  none                                 19  10.65 "    "    77     none   found  none                                 20  14.30 "    "    74     none   found  none                                 21  21.30 "    "    65     none   found  none                                 22  35.50 "    "    45     none   found  none                                 23  56.80 "    "    19     none   found  none                                 24  64.00 "    "    10     none   found  none                                 25  0.00  600  N.sub.2                                                                            92     none   found  none                                 26  0.71  "    "    90     none   found  none                                 27  1.43  "    "    90     none   found  none                                 28  3.55  "    "    87     none   found  none                                 29  4.97  "    "    84     none   found  none                                 30  7.10  "    "    83     none   found  none                                 31  10.65 "    "    76     none   found  none                                 32  14.30 "    "    74     none   found  none                                 33  21.30 "    "    65     none   found  none                                 34  35.50 "    "    45     none   found  none                                 35  56.80 "    "    20     none   found  none                                 36  64.00 "    "    10     none   found  none                                 37  0.00  800  N.sub.2                                                                            91     none   found  none                                 38  0.71  "    "    91     none   found  none                                 39  1.43  "    "    89     none   found  none                                 40  3.55  "    "    86     none   found  none                                 41  4.97  "    "    85     none   found  none                                 42  7.10  "    "    77     none   found  none                                 43  10.65 "    "    60     none   found  none                                 44  14.30 "    "    47     none   found  none                                 45  21.30 "    "    20     none   found  none                                 46  35.50 "    "     2     none   found  none                                 47  56.80 "    "     1     none   found  none                                 48  64.00 "    "     0     none   found  none                                 49  0.00  1100 N.sub.2                                                                            91     found  found  none                                 50  0.71  "    "    91     found  found  none                                 51  1.43  "    "    89     found  found  none                                 52  3.55  "    "    87     found  found  none                                 53  4.97  "    "    84     found  found  none                                 54  7.10  "    "    76     found  found  none                                 55  10.65 "    "    62     found  found  none                                 56  14.30 "    "    45     found  found  none                                 57  21.30 "    "    19     found  found  none                                 58  35.50 "    "     2     found  found  none                                 59  56.80 "    "     1     found  found  none                                 60  64.00 "    "     0     found  found  none                                 61  0.00  1200 N.sub.2                                                                            92     none   found  none                                 62  0.71  "    "    91     none   found  none                                 63  1.43  "    "    90     none   found  none                                 64  3.55  "    "    87     none   found  none                                 65  4.97  "    "    85     none   found  none                                 66  7.10  "    "    74     none   found  none                                 67  10.65 "    "    molten immeasurable                                                                         immeasurable                                                                         immeasurable                         68  14.30 "    "    "      "      "      "                                    69  21.30 "    "    "      "      "      "                                    70  35.50 "    "    "      "      "      "                                    71  56.80 "    "    "      "      "      "                                    72  64.00 "    "    "      "      "      "                                    73  0.00  1450 N.sub.2                                                                            92     none   found  none                                 74  0.71  "    "    90     none   found  none                                 75  1.43  "    "    90     none   found  none                                 76  3.55  "    "    87     none   found  none                                 77  4.97  "    "    84     none   found  none                                 78  7.10  "    "    75     none   found  none                                 79  10.65 "    "    molten immeasurable                                                                         immeasurable                                                                         immeasurable                         80  14.30 "    "    "      "      "      "                                    81  21.30 "    "    "      "      "      "                                    82  35.50 "    "    "      "      "      "                                    83  56.80 "    "    "      "      "      "                                    84  64.00 "    "    "      "      "      "                                    85  0.00  1500 N.sub.2                                                                            86     none   found  none                                 86  0.71  "    "    86     none   found  none                                 87  1.43  "    "    84     none   found  none                                 88  3.55  "    "    80     none   found  none                                 89  4.97  "    "    78     none   found  none                                 90  7.10  "    "    molten immeasurable                                                                         immeasurable                                                                         immeasurable                         91  10.65 "    "    "      "      "      "                                    92  14.30 "    "    "      "      "      "                                    93  21.30 "    "    "      "      "      "                                    94  35.50 "    "    "      "      "      "                                    95  56.80 "    "    "      "      "      "                                    96  64.00 "    "    "      "      "      "                                    __________________________________________________________________________

                  TABLE 4                                                         ______________________________________                                        Firing Experiment Results (TiO.sub.2 : 0 wt. %)                               of the Present Invention                                                                 Amt.              Results of qual.                                 Sam-       of PVA   Heating  anal. of powder                                  ple        added    temp.    X-ray diffractometry                             No.        (wt. %)  T (°C.)                                                                         Fe.sub.3 O.sub.4                                                                    α-Fe.sub.2 O.sub.3                                                             FeO                                 ______________________________________                                        Comp. Ex.                                                                             1      0        800    --    ∘                                                                        --                                "       2      "        1000   --    ∘                                                                        --                                "       3      "        1100   --    ∘                                                                        --                                "       4      "        1150   ∘                                                                       ∘                                                                        --                                "       5      "        1200   ∘                                                                       ∘                                                                        --                                "       6      "        1400   ∘                                                                       ∘                                                                        --                                "       7      "        1450   ∘                                                                       ∘                                                                        --                                "       8      "        1470   ∘                                                                       --     ∘                     Comp. Ex.                                                                             9      2.0      800    ∘                                                                       ∘                                                                        --                                "       10     "        1000   ∘                                                                       ∘                                                                        --                                "       11     "        1100   ∘                                                                       ∘                                                                        --                                "       12     "        1150   ∘                                                                       ∘                                                                        --                                Invention                                                                             13     "        1200   ∘                                                                       --     --                                "       14     "        1400   ∘                                                                       --     --                                "       15     "        1450   ∘                                                                       --     --                                Comp. Ex.                                                                             16     "        1500   ∘                                                                       --     ∘                     Invention                                                                             17     0.1      1300   ∘                                                                       --     --                                "       18     0.2      "      ∘                                                                       --     --                                "       19     0.5      "      ∘                                                                       --     --                                "       20     1.0      "      ∘                                                                       --     --                                "       21     2.0      "      ∘                                                                       --     --                                "       22     3.0      "      ∘                                                                       --     --                                ______________________________________                                    

                                      TABLE 5                                     __________________________________________________________________________    Experimental Examples of the Present Invention (Magnetite, Ti)                    Amt. of Ti                                                                          Heating   Satn.  Grain                                                                              Results of powder X-ray                       Sample                                                                            added temp.                                                                              Atmo-                                                                              magnetization                                                                        strength                                                                           diffractometry                                No. (wt %)                                                                              T (°C.)                                                                     sphere                                                                             δs (emu/g)                                                                     (Pa) α-Fe.sub.2 O.sub.3                                                           Fe.sub.3 O.sub.4                                                                  FeO                                  __________________________________________________________________________    1   0.00  500  N.sub.2                                                                            86     15E6 none found                                                                             none                                 2   0.60  "    "    85     15E6 none found                                                                             none                                 3   1.20  "    "    84     17E6 none found                                                                             none                                 4   3.00  "    "    83     12E6 none found                                                                             none                                 5   6.20  "    "    80     13E6 none found                                                                             none                                 6   9.50  "    "    77     14E6 none found                                                                             none                                 7   13.00 "    "    72     13E6 none found                                                                             none                                 8   16.70 "    "    69     12E6 none found                                                                             none                                 9   28.60 "    "    60     11E6 none found                                                                             none                                 10  37.50 "    "    44     12E6 none found                                                                             none                                 11  52.70 "    "    28     13E6 none found                                                                             none                                 12  84.40 "    "    9      11E6 none found                                                                             none                                 13  0.00  550  N.sub.2                                                                            91     15E8 none found                                                                             none                                 14  0.60  "    "    91     15E8 none found                                                                             none                                 15  1.20  "    "    90     17E8 none found                                                                             none                                 16  3.00  "    "    86     12E8 none found                                                                             none                                 17  6.20  "    "    81     13E8 none found                                                                             none                                 18  9.50  "    "    75     14E8 none found                                                                             none                                 19  13.00 "    "    70     13E8 none found                                                                             none                                 20  16.70 "    "    64     12E8 none found                                                                             none                                 21  28.60 "    "    45     11E8 none found                                                                             none                                 22  37.50 "    "    36     12E8 none found                                                                             none                                 23  52.70 "    "    20     13E8 none found                                                                             none                                 24  84.40 "    "    1      11E8 none found                                                                             none                                 25  0.00  600  N.sub.2                                                                            92     23E8 none found                                                                             none                                 26  0.60  "    "    91     25E8 none found                                                                             none                                 27  1.20  "    "    90     25E8 none found                                                                             none                                 28  3.00  "    "    86     22E8 none found                                                                             none                                 29  6.20  "    "    81     22E8 none found                                                                             none                                 30  9.50  "    "    74     24E8 none found                                                                             none                                 31  13.00 "    "    70     24E8 none found                                                                             none                                 32  16.70 "    "    64     22E8 none found                                                                             none                                 33  28.60 "    "    47     21E8 none found                                                                             none                                 34  37.50 "    "    35     20E8 none found                                                                             none                                 35  52.70 "    "    20     25E8 none found                                                                             none                                 36  84.40 "    "    1      21E8 none found                                                                             none                                 37  0.00  800  N.sub.2                                                                            92     45E8 none found                                                                             none                                 38  0.60  "    "    91     48E8 none found                                                                             none                                 39  1.20  "    "    85     47E8 none found                                                                             none                                 40  3.00  "    "    78     42E8 none found                                                                             none                                 41  6.20  "    "    66     41E8 none found                                                                             none                                 42  9.50  "    "    58     44E8 none found                                                                             none                                 43  13.00 "    "    51     49E8 none found                                                                             none                                 44  16.70 "    "    42     42E8 none found                                                                             none                                 45  28.60 "    "    21     45E8 none found                                                                             none                                 46  37.50 "    "    3      43E8 none found                                                                             none                                 47  52.70 "    "    1      40E8 none found                                                                             none                                 48  84.40 "    "    1      41E8 none found                                                                             none                                 49  0.00  1100 N.sub.2                                                                            91     15E9 none found                                                                             none                                 50  0.60  "    "    91     16E9 none found                                                                             none                                 51  1.20  "    "    87     17E9 none found                                                                             none                                 52  3.00  "    "    77     14E9 none found                                                                             none                                 53  6.20  "    "    58     13E9 none found                                                                             none                                 54  9.50  "    "    35     14E9 none found                                                                             none                                 55  13.00 "    "    20     13E9 none found                                                                             none                                 56  16.70 "    "    2      11E9 none found                                                                             none                                 57  28.60 "    "    2      11E9 none found                                                                             none                                 58  37.50 "    "    2      12E9 none found                                                                             none                                 59  52.70 "    "    1      13E9 none found                                                                             none                                 60  84.40 "    "    0      12E9 none found                                                                             none                                 61  0.00  1200 N.sub.2                                                                            92     25E9 none found                                                                             none                                 62  0.60  "    "    91     25E9 none found                                                                             none                                 63  1.20  "    "    86     23E9 none found                                                                             none                                 64  3.00  "    "    78     22E9 none found                                                                             none                                 65  6.20  "    "    57     22E9 none found                                                                             none                                 66  9.50  "    "    37     25E9 none found                                                                             none                                 67  13.00 "    "    20     24E9 none found                                                                             none                                 68  16.70 "    "    1      23E9 none found                                                                             none                                 69  28.60 "    "    0      21E9 none found                                                                             none                                 70  37.50 "    "    0      20E9 none found                                                                             none                                 71  52.70 "    "    0      26E9 none found                                                                             none                                 72  84.40 "    "    0      21E9 none found                                                                             none                                 73  0.00  1450 N.sub.2                                                                            92     23E9 none found                                                                             none                                 74  0.60  "    "    91     24E9 none found                                                                             none                                 75  1.20  "    "    87     25E9 none found                                                                             none                                 76  3.00  "    "    78     22E9 none found                                                                             none                                 77  6.20  "    "    56     26E9 none found                                                                             none                                 78  9.50  "    "    37     24E9 none found                                                                             none                                 79  13.00 "    "    21     23E9 none found                                                                             none                                 80  16.70 "    "    1      22E9 none found                                                                             none                                 81  28.60 "    "    0      21E9 none found                                                                             none                                 82  37.50 "    "    0      26E9 none found                                                                             none                                 83  52.70 "    "    0      25E9 none found                                                                             none                                 84  84.40 "    "    0      21E9 none found                                                                             none                                 85  0.00  1500 N.sub.2                                                                            85     20E9 none found                                                                             found                                86  0.60  "    "    82     21E9 none found                                                                             found                                87  1.20  "    "    80     21E9 none found                                                                             found                                88  3.00  "    "    69     20E9 none found                                                                             found                                89  6.20  "    "    49     19E9 none found                                                                             found                                90  9.50  "    "    30     22E9 none found                                                                             found                                91  13.00 "    "    14     21E9 none found                                                                             found                                92  16.70 "    "    1      22E9 none found                                                                             found                                93  28.60 "    "    0      18E9 none found                                                                             found                                94  37.50 "    "    0      16E9 none found                                                                             found                                95  52.70 "    "    0      17E9 none found                                                                             found                                96  84.40 "    "    0      19E9 none found                                                                             found                                __________________________________________________________________________

                                      TABLE 6                                     __________________________________________________________________________    Experiment Examples of the Present Invention (1:1 Magnetite + Hematite,       Ti)                                                                               Amt.                                                                          of                    Satn.                                                   mag-                                                                              Amt. of                                                                            Amt.         magnet-                                                 netite                                                                            hematite                                                                           of Ti                                                                             Heating  ization                                                                            Grain                                                                              Results of powder X-ray                   Sample                                                                            added                                                                             added                                                                              added                                                                             temp.                                                                              Atmo-                                                                             δs                                                                           strength                                                                           diffractometry                            No. (wt %)                                                                            (wt %)                                                                             (wt %)                                                                            T (°C.)                                                                     sphere                                                                            (emu/g)                                                                            (Pa) α-Fe.sub.2 O.sub.3                                                           Fe.sub.2 O.sub.4                                                                  FeO                              __________________________________________________________________________    1   50.00                                                                             50.00                                                                              0.00                                                                              1100 N.sub.2                                                                           91   15E9 found                                                                              found                                                                             none                             2   49.70                                                                             49.70                                                                              0.60                                                                              "    "   91   16E9 found                                                                              found                                                                             none                             3   49.40                                                                             49.40                                                                              1.20                                                                              "    "   87   17E9 found                                                                              found                                                                             none                             4   48.50                                                                             48.50                                                                              3.00                                                                              "    "   77   14E9 found                                                                              found                                                                             none                             5   46.90                                                                             46.90                                                                              6.20                                                                              "    "   58   13E9 found                                                                              found                                                                             none                             6   45.25                                                                             45.25                                                                              9.50                                                                              "    "   35   14E9 found                                                                              found                                                                             none                             7   43.50                                                                             43.50                                                                              13.00                                                                             "    "   20   13E9 found                                                                              found                                                                             none                             8   41.70                                                                             41.70                                                                              16.60                                                                             "    "   2    11E9 found                                                                              found                                                                             none                             9   35.70                                                                             35.70                                                                              28.60                                                                             "    "   2    11E9 found                                                                              found                                                                             none                             10  31.30                                                                             31.30                                                                              37.40                                                                             "    "   2    12E9 found                                                                              found                                                                             none                             11  23.70                                                                             23.70                                                                              52.60                                                                             "    "   1    13E9 found                                                                              found                                                                             none                             12  7.80                                                                              7.80 84.40                                                                             "    "   0    12E9 found                                                                              found                                                                             none                             13  50.00                                                                             50.00                                                                              0.00                                                                              1200 N.sub.2                                                                           92   25E9 none found                                                                             none                             14  49.70                                                                             49.70                                                                              0.60                                                                              "    "   91   25E9 none found                                                                             none                             15  49.40                                                                             49.40                                                                              1.20                                                                              "    "   86   23E9 none found                                                                             none                             16  48.50                                                                             48.50                                                                              3.00                                                                              "    "   78   22E9 none found                                                                             none                             17  46.90                                                                             46.90                                                                              6.20                                                                              "    "   57   22E9 none found                                                                             none                             18  45.25                                                                             45.25                                                                              9.50                                                                              "    "   37   25E9 none found                                                                             none                             19  43.50                                                                             43.50                                                                              13.00                                                                             "    "   20   24E9 none found                                                                             none                             20  41.70                                                                             41.70                                                                              16.60                                                                             "    "   1    23E9 none found                                                                             none                             21  35.70                                                                             35.70                                                                              28.60                                                                             "    "   0    21E9 none found                                                                             none                             22  31.30                                                                             31.30                                                                              37.40                                                                             "    "   0    20E9 none found                                                                             none                             23  23.70                                                                             23.70                                                                              52.70                                                                             "    "   0    26E9 none found                                                                             none                             24  7.80                                                                              7.80 84.40                                                                             "    "   0    21E9 none found                                                                             none                             25  50.00                                                                             50.00                                                                              0.00                                                                              1450 N.sub.2                                                                           92   23E9 none found                                                                             none                             26  49.70                                                                             49.70                                                                              0.60                                                                              "    "   91   24E9 none found                                                                             none                             27  49.40                                                                             49.40                                                                              1.20                                                                              "    "   87   25E9 none found                                                                             none                             28  48.50                                                                             48.50                                                                              3.00                                                                              "    "   78   22E9 none found                                                                             none                             29  46.90                                                                             46.90                                                                              6.20                                                                              "    "   56   26E9 none found                                                                             none                             30  45.25                                                                             45.25                                                                              9.50                                                                              "    "   37   24E9 none found                                                                             none                             31  43.50                                                                             43.50                                                                              13.00                                                                             "    "   21   23E9 none found                                                                             none                             32  41.70                                                                             41.70                                                                              16.60                                                                             "    "   1    22E9 none found                                                                             none                             33  35.70                                                                             35.70                                                                              28.60                                                                             "    "   0    21E9 none found                                                                             none                             34  31.30                                                                             31.30                                                                              37.40                                                                             "    "   0    26E9 none found                                                                             none                             35  23.70                                                                             23.70                                                                              52.60                                                                             "    "   0    25E9 none found                                                                             none                             36  7.80                                                                              7.80 84.40                                                                             "    "   0    21E9 none found                                                                             none                             37  50.00                                                                             50.00                                                                              0.00                                                                              1500 N.sub.2                                                                           85   20E9 none found                                                                             found                            38  49.70                                                                             49.70                                                                              0.60                                                                              "    "   82   21E9 none found                                                                             found                            39  49.40                                                                             49.40                                                                              1.20                                                                              "    "   80   21E9 none found                                                                             found                            40  48.50                                                                             48.50                                                                              3.00                                                                              "    "   69   20E9 none found                                                                             found                            41  46.90                                                                             46.90                                                                              6.20                                                                              "    "   49   19E9 none found                                                                             found                            42  45.25                                                                             45.25                                                                              9.50                                                                              "    "   30   22E9 none found                                                                             found                            43  43.50                                                                             43.50                                                                              13.00                                                                             "    "   14   21E9 none found                                                                             found                            44  41.70                                                                             41.70                                                                              16.60                                                                             "    "   1    22E9 none found                                                                             found                            45  35.70                                                                             35.70                                                                              28.60                                                                             "    "   0    18E9 none found                                                                             found                            46  31.30                                                                             31.30                                                                              37.40                                                                             "    "   0    16E9 none found                                                                             found                            47  23.70                                                                             23.70                                                                              52.60                                                                             "    "   0    17E9 none found                                                                             found                            48  7.80                                                                              7.80 84.40                                                                             "    "   0    19E9 none found                                                                             found                            __________________________________________________________________________

                                      TABLE 7                                     __________________________________________________________________________    Experiment Examples of the Present Invention (1:3 Magnetite + Hematite,       Ti)                                                                               Amt.                                                                          of                    Satn.                                                   mag-                                                                              Amt. of                                                                            Amt.         magnet-                                                 netite                                                                            hematite                                                                           of Ti                                                                             Heating  ization                                                                            Grain                                                                              Results of powder X-ray                   Sample                                                                            added                                                                             added                                                                              added                                                                             temp.                                                                              Atmo-                                                                             δs                                                                           strength                                                                           diffractometry                            No. (wt %)                                                                            (wt %)                                                                             (wt %)                                                                            T (°C.)                                                                     sphere                                                                            (emu/g)                                                                            (Pa) α-Fe.sub.2 O.sub.3                                                           Fe.sub.2 O.sub.4                                                                  FeO                              __________________________________________________________________________    1   25.00                                                                             75.00                                                                              0.00                                                                              1100 N.sub.2                                                                           91   15E9 none found                                                                             none                             2   24.90                                                                             74.50                                                                              0.60                                                                              "    "   91   16E9 none found                                                                             none                             3   24.70                                                                             74.10                                                                              1.20                                                                              "    "   87   17E9 none found                                                                             none                             4   24.25                                                                             72.75                                                                              3.00                                                                              "    "   77   14E9 none found                                                                             none                             5   23.45                                                                             70.35                                                                              6.20                                                                              "    "   58   13E9 none found                                                                             none                             6   22.63                                                                             67.87                                                                              9.50                                                                              "    "   35   14E9 none found                                                                             none                             7   21.75                                                                             65.25                                                                              13.00                                                                             "    "   20   13E9 none found                                                                             none                             8   20.83                                                                             62.47                                                                              16.70                                                                             "    "   2    11E9 none found                                                                             none                             9   17.85                                                                             53.55                                                                              28.60                                                                             "    "   2    11E9 none found                                                                             none                             10  15.63                                                                             46.87                                                                              37.50                                                                             "    "   2    12E9 none found                                                                             none                             11  11.83                                                                             35.47                                                                              52.70                                                                             "    "   1    13E9 none found                                                                             none                             12  3.90                                                                              11.70                                                                              84.40                                                                             "    "   0    12E9 none found                                                                             none                             13  25.00                                                                             75.00                                                                              0.00                                                                              1200 N.sub.2                                                                           92   25E9 none found                                                                             none                             14  24.90                                                                             74.50                                                                              0.60                                                                              "    "   91   25E9 none found                                                                             none                             15  24.70                                                                             74.10                                                                              1.20                                                                              "    "   86   23E9 none found                                                                             none                             16  24.25                                                                             72.75                                                                              3.00                                                                              "    "   78   22E9 none found                                                                             none                             17  23.45                                                                             70.35                                                                              6.20                                                                              "    "   57   22E9 none found                                                                             none                             18  22.63                                                                             67.87                                                                              9.50                                                                              "    "   37   25E9 none found                                                                             none                             19  21.75                                                                             65.25                                                                              13.00                                                                             "    "   20   24E9 none found                                                                             none                             20  20.83                                                                             62.47                                                                              16.70                                                                             "    "   1    23E9 none found                                                                             none                             21  17.85                                                                             53.55                                                                              28.60                                                                             "    "   0    21E9 none found                                                                             none                             22  15.63                                                                             46.87                                                                              37.50                                                                             "    "   0    20E9 none found                                                                             none                             23  11.83                                                                             35.47                                                                              52.70                                                                             "    "   0    26E9 none found                                                                             none                             24  3.90                                                                              11.70                                                                              84.40                                                                             "    "   0    21E9 none found                                                                             none                             25  25.00                                                                             75.00                                                                              0.00                                                                              1450 N.sub.2                                                                           92   23E9 none found                                                                             none                             26  24.90                                                                             74.50                                                                              0.60                                                                              "    "   91   24E9 none found                                                                             none                             27  24.70                                                                             74.10                                                                              1.20                                                                              "    "   87   25E9 none found                                                                             none                             28  24.25                                                                             72.75                                                                              3.00                                                                              "    "   78   22E9 none found                                                                             none                             29  23.45                                                                             70.35                                                                              6.20                                                                              "    "   56   26E9 none found                                                                             none                             30  22.63                                                                             67.87                                                                              9.50                                                                              "    "   37   24E9 none found                                                                             none                             31  21.75                                                                             65.25                                                                              13.00                                                                             "    "   21   23E9 none found                                                                             none                             32  20.83                                                                             62.47                                                                              16.70                                                                             "    "   1    22E9 none found                                                                             none                             33  17.85                                                                             53.55                                                                              28.60                                                                             "    "   0    21E9 none found                                                                             none                             34  15.63                                                                             46.87                                                                              37.50                                                                             "    "   0    26E9 none found                                                                             none                             35  11.83                                                                             35.47                                                                              52.70                                                                             "    "   0    25E9 none found                                                                             none                             36  3.90                                                                              11.70                                                                              84.40                                                                             "    "   0    21E9 none found                                                                             none                             37  25.00                                                                             75.00                                                                              0.00                                                                              1500 N.sub.2                                                                           85   20E9 none found                                                                             found                            38  24.90                                                                             74.50                                                                              0.60                                                                              "    "   82   21E9 none found                                                                             found                            39  24.70                                                                             74.10                                                                              1.20                                                                              "    "   80   21E9 none found                                                                             found                            40  24.25                                                                             72.75                                                                              3.00                                                                              "    "   69   20E9 none found                                                                             found                            41  23.45                                                                             70.35                                                                              6.20                                                                              "    "   49   19E9 none found                                                                             found                            42  22.63                                                                             67.87                                                                              9.50                                                                              "    "   30   22E9 none found                                                                             found                            43  21.75                                                                             65.25                                                                              13.00                                                                             "    "   14   21E9 none found                                                                             found                            44  20.83                                                                             62.47                                                                              16.70                                                                             "    "   1    22E9 none found                                                                             found                            45  17.85                                                                             53.55                                                                              28.60                                                                             "    "   0    18E9 none found                                                                             found                            46  15.63                                                                             46.87                                                                              37.50                                                                             "    "   0    16E9 none found                                                                             found                            47  11.83                                                                             35.47                                                                              52.70                                                                             "    "   0    17E9 none found                                                                             found                            48  3.90                                                                              11.70                                                                              84.40                                                                             "    "   0    19E9 none found                                                                             found                            __________________________________________________________________________

                                      TABLE 8                                     __________________________________________________________________________    Experiment Examples of the Present Invention (1:4 Magnetite + Hematite,       Ti)                                                                               Amt.                                                                          of                    Satn.                                                   mag-                                                                              Amt. of                                                                            Amt.         magnet-                                                 netite                                                                            hematite                                                                           of Ti                                                                             Heating  ization                                                                            Grain                                                                              Results of powder X-ray                   Sample                                                                            added                                                                             added                                                                              added                                                                             temp.                                                                              Atmo-                                                                             δs                                                                           strength                                                                           diffractometry                            No. (wt %)                                                                            (wt %)                                                                             (wt %)                                                                            T (°C.)                                                                     sphere                                                                            (emu/g)                                                                            (Pa) α-Fe.sub.2 O.sub.3                                                           Fe.sub.2 O.sub.4                                                                  FeO                              __________________________________________________________________________    1   20.00                                                                             80.00                                                                              0.00                                                                              1100 N.sub.2                                                                           91   15E9 found                                                                              found                                                                             none                             2   19.90                                                                             79.50                                                                              0.60                                                                              "    "   91   16E9 found                                                                              found                                                                             none                             3   19.80                                                                             79.00                                                                              1.20                                                                              "    "   87   17E9 found                                                                              found                                                                             none                             4   19.40                                                                             77.60                                                                              3.00                                                                              "    "   77   14E9 found                                                                              found                                                                             none                             5   18.80                                                                             75.00                                                                              6.20                                                                              "    "   58   13E9 found                                                                              found                                                                             none                             6   18.10                                                                             72.40                                                                              9.50                                                                              "    "   35   14E9 found                                                                              found                                                                             none                             7   17.40                                                                             69.40                                                                              13.00                                                                             "    "   20   13E9 found                                                                              found                                                                             none                             8   16.70                                                                             66.60                                                                              16.70                                                                             "    "   2    11E9 found                                                                              found                                                                             none                             9   14.30                                                                             57.10                                                                              28.60                                                                             "    "   2    11E9 found                                                                              found                                                                             none                             10  12.50                                                                             50.00                                                                              37.50                                                                             "    "   2    12E9 found                                                                              found                                                                             none                             11  9.50                                                                              37.80                                                                              52.70                                                                             "    "   1    13E9 found                                                                              found                                                                             none                             12  3.10                                                                              12.50                                                                              84.40                                                                             "    "   0    12E9 found                                                                              found                                                                             none                             13  20.00                                                                             80.00                                                                              0.00                                                                              1200 N.sub.2                                                                           92   25E9 none found                                                                             none                             14  19.90                                                                             79.50                                                                              0.60                                                                              "    "   91   25E9 none found                                                                             no)ne                            15  19.80                                                                             79.00                                                                              1.20                                                                              "    "   86   23E9 none found                                                                             none                             16  19.40                                                                             77.60                                                                              3.00                                                                              "    "   78   22E9 none found                                                                             none                             17  18.80                                                                             75.00                                                                              6.20                                                                              "    "   57   22E9 none found                                                                             none                             18  18.10                                                                             72.40                                                                              9.50                                                                              "    "   37   25E9 none found                                                                             none                             19  17.40                                                                             69.40                                                                              13.00                                                                             "    "   20   24E9 none found                                                                             none                             20  16.70                                                                             66.60                                                                              16.70                                                                             "    "   1    23E9 none found                                                                             none                             21  14.30                                                                             57.10                                                                              28.60                                                                             "    "   0    21E9 none found                                                                             none                             22  12.50                                                                             50.00                                                                              37.50                                                                             "    "   0    20E9 none found                                                                             none                             23  9.50                                                                              37.80                                                                              52.70                                                                             "    "   0    26E9 none found                                                                             none                             24  3.10                                                                              12.50                                                                              84.40                                                                             "    "   0    21E9 none found                                                                             none                             25  20.00                                                                             80.00                                                                              0.00                                                                              1450 N.sub.2                                                                           92   23E9 none found                                                                             none                             26  19.90                                                                             79.50                                                                              0.60                                                                              "    "   91   24E9 none found                                                                             none                             27  19.80                                                                             79.00                                                                              1.20                                                                              "    "   87   25E9 none found                                                                             none                             28  19.40                                                                             77.60                                                                              3.00                                                                              "    "   78   22E9 none found                                                                             none                             29  18.80                                                                             75.00                                                                              6.20                                                                              "    "   56   26E9 none found                                                                             none                             30  18.10                                                                             72.40                                                                              9.50                                                                              "    "   37   24E9 none found                                                                             none                             31  17.40                                                                             69.40                                                                              13.00                                                                             "    "   21   23E9 none found                                                                             none                             32  16.70                                                                             66.60                                                                              16.70                                                                             "    "   1    22E9 none found                                                                             none                             33  14.30                                                                             57.10                                                                              28.60                                                                             "    "   0    21E9 none found                                                                             none                             34  12.50                                                                             50.00                                                                              37.50                                                                             "    "   0    26E9 none found                                                                             none                             35  9.50                                                                              37.80                                                                              52.70                                                                             "    "   0    25E9 none found                                                                             none                             36  3.10                                                                              12.50                                                                              84.40                                                                             "    "   0    21E9 none found                                                                             none                             37  20.00                                                                             80.00                                                                              0.00                                                                              1500 N.sub.2                                                                           85   20E9 none found                                                                             found                            38  19.90                                                                             79.50                                                                              0.60                                                                              "    "   82   21E9 none found                                                                             found                            39  19.80                                                                             79.00                                                                              1.20                                                                              "    "   80   21E9 none found                                                                             found                            40  19.40                                                                             77.60                                                                              3.00                                                                              "    "   69   20E9 none found                                                                             found                            41  18.80                                                                             75.00                                                                              6.20                                                                              "    "   49   19E9 none found                                                                             found                            42  18.10                                                                             72.40                                                                              9.50                                                                              "    "   30   22E9 none found                                                                             found                            43  17.40                                                                             69.40                                                                              13.00                                                                             "    "   14   21E9 none found                                                                             found                            44  16.70                                                                             66.60                                                                              16.70                                                                             "    "   1    22E9 none found                                                                             found                            45  14.30                                                                             57.10                                                                              28.60                                                                             "    "   0    18E9 none found                                                                             found                            46  12.50                                                                             50.00                                                                              37.50                                                                             "    "   0    16E9 none found                                                                             found                            47  9.50                                                                              37.80                                                                              52.70                                                                             "    "   0    17E9 none found                                                                             found                            48  3.10                                                                              12.50                                                                              84.40                                                                             "    "   0    19E9 none found                                                                             found                            __________________________________________________________________________

                                      TABLE 9                                     __________________________________________________________________________    Other Experimental Examples of the Present Invention (Hematite, Sn)                               Satn.                                                         Amt.            magnet-                                                       of Sn                                                                             Heating     ization                                                                            Results of powder                                    Sample                                                                            added                                                                             temp.       δs                                                                           X-ray diffractometry                                 No. (wt %)                                                                            T (°C.)                                                                     Atmosphere                                                                           (emu/g)                                                                            α-Fe.sub.2 O.sub.3                                                             Fe.sub.3 O.sub.4                                                                   FeO                                      __________________________________________________________________________    1   0.00                                                                              1100 N.sub.2                                                                              86   found  found                                                                              none                                     2   0.80                                                                              "    "      85   found  found                                                                              none                                     3   1.58                                                                              "    "      84   found  found                                                                              none                                     4   3.98                                                                              "    "      83   found  found                                                                              none                                     5   8.04                                                                              "    "      75   found  found                                                                              none                                     6   16.44                                                                             "    "      69   found  found                                                                              none                                     7   44.04                                                                             "    "      41   found  found                                                                              none                                     8   64.74                                                                             "    "      26   found  found                                                                              none                                     9   76.00                                                                             "    "      16   found  found                                                                              none                                     10  87.60                                                                             "    "      8    found  found                                                                              none                                     11  0.00                                                                              1200 N.sub.2                                                                              92   none   found                                                                              none                                     12  0.80                                                                              "    "      91   none   found                                                                              none                                     13  1.58                                                                              "    "      90   none   found                                                                              none                                     14  3.98                                                                              "    "      87   none   found                                                                              none                                     15  8.04                                                                              "    "      82   none   found                                                                              none                                     16  16.44                                                                             "    "      76   none   found                                                                              none                                     17  44.04                                                                             "    "      47   none   found                                                                              none                                     18  64.74                                                                             "    "      28   none   found                                                                              none                                     19  76.00                                                                             "    "      21   none   found                                                                              none                                     20  87.60                                                                             "    "      9    none   found                                                                              none                                     21  0.00                                                                              1300 N.sub.2                                                                              92   none   found                                                                              none                                     22  0.80                                                                              "    "      91   none   found                                                                              none                                     23  1.58                                                                              "    "      90   none   found                                                                              none                                     24  3.98                                                                              "    "      86   none   found                                                                              none                                     25  8.04                                                                              "    "      82   none   found                                                                              none                                     26  16.44                                                                             "    "      74   none   found                                                                              none                                     27  44.04                                                                             "    "      47   none   found                                                                              none                                     28  64.74                                                                             "    "      25   none   found                                                                              none                                     29  76.00                                                                             "    "      21   none   found                                                                              none                                     30  87.60                                                                             "    "      8    none   found                                                                              none                                     31  0.00                                                                              1450 N.sub.2                                                                              92   none   found                                                                              none                                     32  0.80                                                                              "    "      91   none   found                                                                              none                                     33  1.58                                                                              "    "      90   none   found                                                                              none                                     34  3.98                                                                              "    "      87   none   found                                                                              none                                     35  8.04                                                                              "    "      85   none   found                                                                              none                                     36  16.44                                                                             "    "      75   none   found                                                                              none                                     37  44.04                                                                             "    "      47   none   found                                                                              none                                     38  64.74                                                                             "    "      29   none   found                                                                              none                                     39  76.00                                                                             "    "      21   none   found                                                                              none                                     40  87.60                                                                             "    "      9    none   found                                                                              none                                     41  0.00                                                                              1500 N.sub.2                                                                              86   none   found                                                                              found                                    42  0.80                                                                              "    "      85   none   found                                                                              found                                    43  1.58                                                                              "    "      84   none   found                                                                              found                                    44  3.98                                                                              "    "      83   none   found                                                                              found                                    45  8.04                                                                              "    "      75   none   found                                                                              found                                    46  16.44                                                                             "    "      68   none   found                                                                              found                                    47  44.04                                                                             "    "      39   none   found                                                                              found                                    48  64.74                                                                             "    "      22   none   found                                                                              found                                    49  76.00                                                                             "    "      15   none   found                                                                              found                                    50  87.60                                                                             "    "      2    none   found                                                                              found                                    __________________________________________________________________________

                                      TABLE 10                                    __________________________________________________________________________    Other Experimental Examples of the Present Invention                          (1:1 Hematite + Magnetite, Sn)                                                                    Satn.                                                         Amt.            magnet-                                                       of Sn                                                                             Heating     ization                                                                            Results of powder                                    Sample                                                                            added                                                                             temp.       δs                                                                           X-ray diffractometry                                 No. (wt %)                                                                            T (°C.)                                                                     Atmosphere                                                                           (emu/g)                                                                            α-Fe.sub.2 O.sub.3                                                             Fe.sub.3 O.sub.4                                                                   FeO                                      __________________________________________________________________________    1   0.00                                                                              1100 N.sub.2                                                                              86   found  found                                                                              none                                     2   0.80                                                                              "    "      85   found  found                                                                              none                                     3   1.58                                                                              "    "      84   found  found                                                                              none                                     4   3.98                                                                              "    "      83   found  found                                                                              none                                     5   8.04                                                                              "    "      75   found  found                                                                              none                                     6   16.44                                                                             "    "      69   found  found                                                                              none                                     7   44.04                                                                             "    "      39   found  found                                                                              none                                     8   64.74                                                                             "    "      22   found  found                                                                              none                                     9   76.00                                                                             "    "      16   found  found                                                                              none                                     10  87.60                                                                             "    "      4    found  found                                                                              none                                     11  0.00                                                                              1200 N.sub.2                                                                              92   none   found                                                                              none                                     12  0.80                                                                              "    "      91   none   found                                                                              none                                     13  1.58                                                                              "    "      90   none   found                                                                              none                                     14  3.98                                                                              "    "      87   none   found                                                                              none                                     15  8.04                                                                              "    "      85   none   found                                                                              none                                     16  16.44                                                                             "    "      75   none   found                                                                              none                                     17  44.04                                                                             "    "      47   none   found                                                                              none                                     18  64.74                                                                             "    "      29   none   found                                                                              none                                     19  76.00                                                                             "    "      21   none   found                                                                              none                                     20  87.60                                                                             "    "      9    none   found                                                                              none                                     21  0.00                                                                              1300 N.sub.2                                                                              92   none   found                                                                              none                                     22  0.80                                                                              "    "      91   none   found                                                                              none                                     23  1.58                                                                              "    "      90   none   found                                                                              none                                     24  3.98                                                                              "    "      87   none   found                                                                              none                                     25  8.04                                                                              "    "      85   none   found                                                                              none                                     26  16.44                                                                             "    "      74   none   found                                                                              none                                     27  44.04                                                                             "    "      47   none   found                                                                              none                                     28  64.74                                                                             "    "      29   none   found                                                                              none                                     29  76.00                                                                             "    "      20   none   found                                                                              none                                     30  87.60                                                                             "    "      9    none   found                                                                              none                                     31  0.00                                                                              1450 N.sub.2                                                                              92   none   found                                                                              none                                     32  0.80                                                                              "    "      91   none   found                                                                              none                                     33  1.58                                                                              "    "      90   none   found                                                                              none                                     34  3.98                                                                              "    "      86   none   found                                                                              none                                     35  8.04                                                                              "    "      85   none   found                                                                              none-                                    36  16.44                                                                             "    "      74   none   found                                                                              none                                     37  44.04                                                                             "    "      46   none   found                                                                              none                                     38  64.74                                                                             "    "      29   none   found                                                                              none                                     39  76.00                                                                             "    "      20   none   found                                                                              none                                     40  87.60                                                                             "    "      10   none   found                                                                              none                                     41  0.00                                                                              1500 N.sub.2                                                                              86   none   found                                                                              found                                    42  0.80                                                                              "    "      85   none   found                                                                              found                                    43  1.58                                                                              "    "      84   none   found                                                                              found                                    44  3.98                                                                              "    "      83   none   found                                                                              found                                    45  8.04                                                                              "    "      75   none   found                                                                              found                                    46  16.44                                                                             "    "      69   none   found                                                                              found                                    47  44.04                                                                             "    "      39   none   found                                                                              found                                    48  64.74                                                                             "    "      22   none   found                                                                              found                                    49  76.00                                                                             "    "      16   none   found                                                                              found                                    50  87.60                                                                             "    "      4    none   found                                                                              found                                    __________________________________________________________________________

                                      TABLE 11                                    __________________________________________________________________________    Other Experimental Examples of the Present Invention                          (Magnetite, Sn)                                                                                   Satn.                                                         Amt.            magnet-                                                       of Sn                                                                             Heating     ization                                                                            Results of powder                                    Sample                                                                            added                                                                             temp.       δs                                                                           X-ray diffractometry                                 No. (wt %)                                                                            T (°C.)                                                                     Atmosphere                                                                           (emu/g)                                                                            α-Fe.sub.2 O.sub.3                                                             Fe.sub.3 O.sub.4                                                                   FeO                                      __________________________________________________________________________    1   0.00                                                                              500  N.sub.2                                                                              86   found  found                                                                              none                                     2   0.80                                                                              "    "      85   found  found                                                                              none                                     3   1.58                                                                              "    "      84   found  found                                                                              none                                     4   3.98                                                                              "    "      83   found  found                                                                              none                                     5   8.04                                                                              "    "      75   found  found                                                                              none                                     6   16.44                                                                             "    "      70   found  found                                                                              none                                     7   44.04                                                                             "    "      39   found  found                                                                              none                                     8   64.74                                                                             "    "      22   found  found                                                                              none                                     9   76.00                                                                             "    "      14   found  found                                                                              none                                     10  87.60                                                                             "    "      3    found  found                                                                              none                                     11  0.00                                                                              550  N.sub.2                                                                              92   none   found                                                                              none                                     12  0.80                                                                              "    "      91   none   found                                                                              none                                     13  1.58                                                                              "    "      90   none   found                                                                              none                                     14  3.98                                                                              "    "      87   none   found                                                                              none                                     15  8.04                                                                              "    "      85   none   found                                                                              none                                     16  16.44                                                                             "    "      75   none   found                                                                              none                                     17  44.04                                                                             "    "      47   none   found                                                                              none                                     18  64.74                                                                             "    "      29   none   found                                                                              none                                     19  76.00                                                                             "    "      21   none   found                                                                              none                                     20  87.60                                                                             "    "      9    none   found                                                                              none                                     21  0.00                                                                              600  N.sub.2                                                                              92   none   found                                                                              none                                     22  0.80                                                                              "    "      91   none   found                                                                              none                                     23  1.58                                                                              "    "      90   none   found                                                                              none                                     24  3.98                                                                              "    "      87   none   found                                                                              none                                     25  8.04                                                                              "    "      85   none   found                                                                              none                                     26  16.44                                                                             "    "      74   none   found                                                                              none                                     27  44.04                                                                             "    "      47   none   found                                                                              none                                     28  64.74                                                                             "    "      29   none   found                                                                              none                                     29  76.00                                                                             "    "      20   none   found                                                                              none                                     30  87.60                                                                             "    "      9    none   found                                                                              none                                     31  0.00                                                                              800  N.sub.2                                                                              92   none   found                                                                              none                                     32  0.80                                                                              "    "      91   none   found                                                                              none.                                    33  1.58                                                                              "    "      90   none   found                                                                              none                                     34  3.98                                                                              "    "      86   none   found                                                                              none                                     35  8.04                                                                              "    "      85   none   found                                                                              none                                     36  16.44                                                                             "    "      74   none   found                                                                              none                                     37  44.04                                                                             "    "      46   none   found                                                                              none                                     38  64.74                                                                             "    "      29   none   found                                                                              none                                     39  76.00                                                                             "    "      20   none   found                                                                              none                                     40  87.60                                                                             "    "      10   none   found                                                                              none                                     41  0.00                                                                              1100 N.sub.2                                                                              92   none   found                                                                              none                                     42  0.80                                                                              "    "      91   none   found                                                                              none                                     43  1.58                                                                              "    "      90   none   found                                                                              none                                     44  3.98                                                                              "    "      87   none   found                                                                              none                                     45  8.04                                                                              "    "      83   none   found                                                                              none                                     46  16.44                                                                             "    "      75   none   found                                                                              none                                     47  44.04                                                                             "    "      47   none   found                                                                              none                                     48  64.74                                                                             "    "      28   none   found                                                                              none                                     49  76.00                                                                             "    "      20   none   found                                                                              none                                     50  87.60                                                                             "    "      9    none   found                                                                              none                                     51  0.00                                                                              1200 N.sub.2                                                                              92   none   found                                                                              none                                     52  0.80                                                                              "    "      91   none   found                                                                              none                                     53  1.58                                                                              "    "      90   none   found                                                                              none                                     54  3.98                                                                              "    "      87   none   found                                                                              none                                     55  8.04                                                                              "    "      82   none   found                                                                              none                                     56  16.44                                                                             "    "      74   none   found                                                                              none                                     57  44.04                                                                             "    "      47   none   found                                                                              none                                     58  64.74                                                                             "    "      26   none   found                                                                              none                                     59  76.00                                                                             "    "      21   none   found                                                                              none                                     60  87.60                                                                             "    "      8    none   found                                                                              none                                     61  0.00                                                                              1300 N.sub.2                                                                              92   none   found                                                                              none                                     62  0.80                                                                              "    "      91   none   found                                                                              none                                     63  1.58                                                                              "    "      90   none   found                                                                              none                                     64  3.98                                                                              "    "      87   none   found                                                                              none                                     65  8.04                                                                              "    "      85   none   found                                                                              none                                     66  16.44                                                                             "    "      74   none   found                                                                              none                                     67  44.04                                                                             "    "      47   none   found                                                                              none                                     68  64.74                                                                             "    "      29   none   found                                                                              none                                     69  76.00                                                                             "    "      20   none   found                                                                              none                                     70  87.60                                                                             "    "      9    none   found                                                                              none                                     71  0.00                                                                              1450 N.sub.2                                                                              92   none   found                                                                              none                                     72  0.80                                                                              "    "      91   none   found                                                                              none                                     73  1.58                                                                              "    "      90   none   found                                                                              none                                     74  3.98                                                                              "    "      87   none   found                                                                              none                                     75  8.04                                                                              "    "      82   none   found                                                                              none                                     76  16.44                                                                             "    "      74   none   found                                                                              none                                     77  44.04                                                                             "    "      47   none   found                                                                              none                                     78  64.74                                                                             "    "      26   none   found                                                                              none                                     79  76.00                                                                             "    "      21   none   found                                                                              none                                     80  87.60                                                                             "    "      8    none   found                                                                              none                                     81  0.00                                                                              1500 N.sub.2                                                                              86   none   found                                                                              found                                    82  0.80                                                                              "    "      85   none   found                                                                              found                                    83  1.58                                                                              "    "      84   none   found                                                                              found                                    84  3.98                                                                              "    "      83   none   found                                                                              found                                    85  8.04                                                                              "    "      75   none   found                                                                              found                                    86  16.44                                                                             "    "      69   none   found                                                                              found                                    87  44.04                                                                             "    "      39   none   found                                                                              found                                    88  64.74                                                                             "    "      22   none   found                                                                              found                                    89  76.00                                                                             "    "      16   none   found                                                                              found                                    90  87.60                                                                             "    "      4    none   found                                                                              found                                    __________________________________________________________________________

                  TABLE 12                                                        ______________________________________                                        Firing Experiment Results (SiO.sub.2 : O wt. %)                               of the Present Invention                                                                 Amt.                                                               Sam-       of PVA   Heating  Results of powder                                ple        added    temp.    X-ray diffractometry                             No.        (wt. %)  T (°C.)                                                                         Fe.sub.3 O.sub.4                                                                    α-Fe.sub.2 O.sub.3                                                             FeO                                 ______________________________________                                        Comp. Ex.                                                                             1      0         800   --    ∘                                                                        --                                "       2      "        1000   --    ∘                                                                        --                                "       3      "        1100   --    ∘                                                                        --                                "       4      "        1150   ∘                                                                       ∘                                                                        --                                "       5      "        1200   ∘                                                                       ∘                                                                        --                                "       6      "        1400   ∘                                                                       ∘                                                                        --                                "       7      "        1450   ∘                                                                       ∘                                                                        --                                "       8      "        1470   ∘                                                                       --     ∘                     Comp. Ex.                                                                             9      2.0       800   ∘                                                                       ∘                                                                        --                                "       10     "        1000   ∘                                                                       ∘                                                                        --                                "       11     "        1100   ∘                                                                       ∘                                                                        --                                "       12     "        1150   ∘                                                                       ∘                                                                        --                                Invention                                                                             13     "         200   ∘                                                                       --     --                                "       14     "        1400   ∘                                                                       --     --                                "       15     "        1450   ∘                                                                       --     --                                Comp. Ex.                                                                             16     "        1500   ∘                                                                       --     ∘                     Invention                                                                             17     0.1      1300   ∘                                                                       --     --                                "       18     0.2      "      ∘                                                                       --     --                                "       19     0.5      "      ∘                                                                       --     --                                "       20     1.0      "      ∘                                                                       --     --                                "       21     2.0      "      ∘                                                                       --     --                                "       22     3.0      "      ∘                                                                       --     --                                ______________________________________                                    

                                      TABLE 13                                    __________________________________________________________________________    Experimental Examples of the Present Invention (Hematite)                         Amt. of Si                                                                          Heating  Satn.                                                      Sample                                                                            added temp.                                                                              Atmo-                                                                             magnetization                                                                        Results of powder X-ray diffractometry              No. (wt %)                                                                              T (°C.)                                                                     sphere                                                                            δs (emu/g)                                                                     α-Fe.sub.2 O.sub.3                                                             Fe.sub.3 O.sub.4                                                                    FeO                                    __________________________________________________________________________    1   0.00  1100 N.sub.2                                                                           86     found  found none                                   2   0.50  "    "   85     found  found none                                   3   0.90  "    "   84     found  found none                                   4   2.40  "    "   83     found  found none                                   5   4.90  "    "   75     found  found none                                   6   10.40 "    "   69     found  found none                                   7   31.50 "    "   39     found  found none                                   8   51.20 "    "   22     found  found none                                   9   65.00 "    "   16     found  found none                                   10  79.50 "    "   4      found  found none                                   11  0.00  1200 N.sub.2                                                                           92     none   found none                                   12  0.50  "    "   91     none   found none                                   13  0.90  "    "   90     none   found none                                   14  2.40  "    "   87     none   found none                                   15  4.90  "    "   83     none   found none                                   16  10.40 "    "   75     none   found none                                   17  31.50 "    "   47     none   found none                                   18  51.20 "    "   28     none   found none                                   19  65.00 "    "   20     none   found none                                   20  79.50 "    "   9      none   found none                                   21  0.00  1300 N.sub.2                                                                           92     none   found none                                   22  0.50  "    "   91     none   found none                                   23  0.90  "    "   90     none   found none                                   24  2.40  "    "   87     none   found none                                   25  4.90  "    "   82     none   found none                                   26  10.40 "    "   74     none   found none                                   27  31.50 "    "   47     none   found none                                   28  51.20 "    "   26     none   found none                                   29  65.00 "    "   21     none   found none                                   30  79.50 "    "   8      none   found none                                   31  0.00  1450 N.sub.2                                                                           92     none   found none                                   32  0.50  "    "   91     none   found none                                   33  0.90  "    "   90     none   found none                                   34  2.40  "    "   87     none   found none                                   35  4.90  "    "   85     none   found none                                   36  10.40 "    "   74     none   found none                                   37  31.50 "    "   47     none   found none                                   38  51.20 "    "   29     none   found none                                   39  65.00 "    "   20     none   found none                                   40  79.50 "    "   9      none   found none                                   41  0.00  1500 N.sub.2                                                                           86     none   found found                                  42  0.50  "    "   85     none   found found                                  43  0.90  "    "   84     none   found found                                  44  2.40  "    "   83     none   found found                                  45  4.90  "    "   75     none   found found                                  46  10.40 "    "   69     none   found found                                  47  31.50 "    "   39     none   found found                                  48  51.20 "    "   22     none   found found                                  49  65.00 "    "   16     none   found found                                  50  79.50 "    "   4      none   found found                                  __________________________________________________________________________

                                      TABLE 14                                    __________________________________________________________________________    Experimental Examples of the Present Invention (Hematite + Magnetite)             Amt. of Si                                                                          Heating  Satn.                                                      Sample                                                                            added temp.                                                                              Atmo-                                                                             magnetization                                                                        Results of powder X-ray diffractometry              No. (wt %)                                                                              T (°C.)                                                                     sphere                                                                            δs (emu/g)                                                                     α-Fe.sub.2 O.sub.3                                                             Fe.sub.3 O.sub.4                                                                    FeO                                    __________________________________________________________________________    1   0.00  1100 N.sub.2                                                                           86     found  found none                                   2   0.50  "    "   85     found  found none                                   3   0.90  "    "   84     found  found none                                   4   2.40  "    "   83     found  found none                                   5   4.90  "    "   75     found  found none                                   6   10.40 "    "   69     found  found none                                   7   31.50 "    "   39     found  found none                                   8   51.20 "    "   22     found  found none                                   9   65.00 "    "   16     found  found none                                   10  79.50 "    "   4      found  found none                                   11  0.00  1200 N.sub.2                                                                           92     none   found none                                   12  0.50  "    "   91     none   found none                                   13  0.90  "    "   90     none   found none                                   14  2.40  "    "   87     none   found none                                   15  4.90  "    "   85     none   found none                                   16  10.40 "    "   75     none   found none                                   17  31.50 "    "   47     none   found none                                   18  51.20 "    "   29     none   found none                                   19  65.00 "    "   21     none   found none                                   20  79.50 "    "   9      none   found none                                   21  0.00  1300 N.sub.2                                                                           92     none   found none                                   22  0.50  "    "   91     none   found none                                   23  0.90  "    "   90     none   found none                                   24  2.40  "    "   87     none   found none                                   25  4.90  "    "   85     none   found none                                   26  10.40 "    "   74     none   found none                                   27  31.50 "    "   47     none   found none                                   28  51.20 "    "   29     none   found none                                   29  65.00 "    "   20     none   found none                                   30  79.50 "    "   9      none   found none                                   31  0.00  1450 N.sub.2                                                                           92     none   found none                                   32  0.50  "    "   91     none   found none                                   33  0.90  "    "   90     none   found none                                   34  2.40  "    "   86     none   found none                                   35  4.90  "    "   85     none   found none                                   36  10.40 "    "   74     none   found none-                                  37  31.50 "    "   46     none   found none                                   38  51.20 "    "   29     none   found none                                   39  65.00 "    "   20     none   found none                                   40  79.50 "    "   10     none   found none                                   41  0.00  1500 N.sub.2                                                                           86     none   found found                                  42  0.50  "    "   85     none   found found                                  43  0.90  "    "   84     none   found found                                  44  2.40  "    "   83     none   found found                                  45  4.90  "    "   75     none   found found                                  46  10.40 "    "   69     none   found found                                  47  31.50 "    "   39     none   found found                                  48  51.20 "    "   22     none   found found                                  49  65.00 "    "   16     none   found found                                  50  79.50 "    "   4      none   found found                                  __________________________________________________________________________

                                      TABLE 15                                    __________________________________________________________________________    Experimental Examples of the Present Invention (Magnetite)                        Amt. of Si                                                                          Heating  Satn.                                                      Sample                                                                            added temp.                                                                              Atmo-                                                                             magnetization                                                                        Results of powder X-ray diffractometry              No. (wt %)                                                                              T (°C.)                                                                     sphere                                                                            δs (emu/g)                                                                     α-Fe.sub.2 O.sub.3                                                             Fe.sub.3 O.sub.4                                                                    FeO                                    __________________________________________________________________________    1   0.00  500  N.sub.2                                                                           86     found  found none                                   2   0.50  "    "   85     found  found none                                   3   0.90  "    "   84     found  found none                                   4   2.40  "    "   83     found  found none                                   5   4.90  "    "   75     found  found none                                   6   10.40 "    "   70     found  found none                                   7   31.50 "    "   39     found  found none                                   8   51.20 "    "   22     found  found none                                   9   65.00 "    "   14     found  found none                                   10  79.50 "    "   3      found  found none                                   11  0.00  550  N.sub.2                                                                           92     none   found none                                   12  0.50  "    "   91     none   found none                                   13  0.90  "    "   90     none   found none                                   14  2.40  "    "   87     none   found none                                   15  4.90  "    "   85     none   found none                                   16  10.40 "    "   75     none   found none                                   17  31.50 "    "   47     none   found none                                   18  51.20 "    "   29     none   found none                                   19  65.00 "    "   21     none   found none                                   20  79.50 "    "   9      none   found none                                   21  0.00  600  N.sub.2                                                                           92     none   found none                                   22  0.50  "    "   91     none   found none                                   23  0.90  "    "   90     none   found none                                   24  2.40  "    "   87     none   found none                                   25  4.90  "    "   85     none   found none                                   26  10.40 "    "   74     none   found none                                   27  31.50 "    "   47     none   found none                                   28  51.20 "    "   29     none   found none                                   29  65.00 "    "   20     none   found none                                   30  79.50 "    "   9      none   found none                                   31  0.00  800  N.sub.2                                                                           92     none   found none                                   32  0.50  "    "   91     none   found none                                   33  0.90  "    "   90     none   found none                                   34  2.40  "    "   86     none   found none                                   35  4.90  "    "   85     none   found none                                   36  10.40 "    "   74     none   found none                                   37  31.50 "    "   46     none   found none                                   38  51.20 "    "   29     none   found none                                   39  65.00 "    "   20     none   found none                                   40  79.50 "    "   10     none   found none                                   41  0.00  1100 N.sub.2                                                                           92     none   found none                                   42  0.50  "    "   91     none   found none                                   43  0.90  "    "   90     none   found none                                   44  2.40  "    "   87     none   found none                                   45  4.90  "    "   83     none   found none                                   46  10.40 "    "   75     none   found none                                   47  31.50 "    "   47     none   found none                                   48  51.20 "    "   28     none   found none                                   49  65.00 "    "   20     none   found none                                   50  79.50 "    "   9      none   found none                                   51  0.00  1200 N.sub.2                                                                           92     none   found none                                   52  0.50  "    "   91     none   found none                                   53  0.90  "    "   90     none   found none                                   54  2.40  "    "   87     none   found none                                   55  4.90  "    "   82     none   found none                                   56  10.40 "    "   74     none   found none                                   57  31.50 "    "   47     none   found none                                   58  51.20 "    "   26     none   found none                                   59  65.00 "    "   21     none   found none                                   60  79.50 "    "   8      none   found none                                   61  0.00  1300 N.sub.2                                                                           92     none   found none                                   62  0.50  "    "   91     none   found none                                   63  0.90  "    "   90     none   found none                                   64  2.40  "    "   87     none   found none                                   65  4.90  "    "   85     none   found none                                   66  10.40 "    "   74     none   found none                                   67  31.50 "    "   47     none   found none                                   68  51.20 "    "   29     none   found none                                   69  65.00 "    "   20     none   found none                                   70  79.50 "    "   9      none   found none                                   71  0.00  1450 N.sub.2                                                                           92     none   found none                                   72  0.50  "    "   91     none   found none                                   73  0.90  "    "   90     none   found none                                   74  2.40  "    "   87     none   found none                                   75  4.90  "    "   82     none   found none                                   76  10.40 "    "   74     none   found none                                   77  31.50 "    "   47     none   found none                                   78  51.20 "    "   26     none   found none                                   79  65.00 "    "   21     none   found none                                   80  79.50 "    "   8      none   found none                                   81  0.00  1500 N.sub.2                                                                           86     none   found found                                  82  0.50  "    "   85     none   found found                                  83  0.90  "    "   84     none   found found                                  84  2.40  "    "   83     none   found found                                  85  4.90  "    "   75     none   found found                                  86  10.40 "    "   69     none   found found                                  87  31.50 "    "   39     none   found found                                  88  51.20 "    "   22     none   found found                                  89  65.00 "    "   16     none   found found                                  90  79.50 "    "   4      none   found found                                  __________________________________________________________________________

                  TABLE 16                                                        ______________________________________                                        Firing Experiment Results (Hematite + Mg)                                     of the Present Invention                                                            Amt. of Mg                                                                              Heating        Satn.    Grain                                 Sample                                                                              added     temp.    Atmos-                                                                              magnetization                                                                          strength                              No.   (wt %)    (°C.)                                                                           phere δs (emu/g)                                                                       (Pa)                                  ______________________________________                                        1     0.00      500      N.sub.2                                                                             40       12E6                                  2     0.10      "        "     41       15E6                                  3     0.20      "        "     41       14E6                                  4     0.80      "        "     35       13E6                                  5     1.60      "        "     29       13E6                                  6     3.60      "        "     27       12E6                                  7     13.20     "        "     16       14E6                                  8     18.50     "        "     10       14E6                                  9     26.20     "        "     5        12E6                                  10    40.00     "        "     1        11E6                                  11    0.00      550      N.sub.2                                                                             40       12E8                                  12    0.10      "        "     41       13E8                                  13    0.20      "        "     41       12E8                                  14    0.80      "        "     35       13E8                                  15    1.60      "        "     29       14E8                                  16    3.60      "        "     27       13E8                                  17    13.20     "        "     16       15E8                                  18    18.50     "        "     10       14E8                                  19    26.20     "        "     5        15E8                                  20    40.00     "        "     1        13E8                                  21    0.00      600      N.sub.2                                                                             40       15E8                                  22    0.10      "        "     41       15E8                                  23    0.20      "        "     41       16E8                                  24    0.80      "        "     35       15E8                                  25    1.60      "        "     28       14E8                                  26    3.60      "        "     27       16E8                                  27    13.20     "        "     16       15E8                                  28    18.50     "        "     11       16E8                                  29    26.20     "        "     5        15E8                                  30    40.00     "        "     1        17E8                                  31    0.00      800      N.sub.2                                                                             40       25E8                                  32    0.10      "        "     41       26E8                                  33    0.20      "        "     41       26E8                                  34    0.80      "        "     35       26E8                                  35    1.60      "        "     29       25E8                                  36    3.60      "        "     26       28E8                                  37    13.20     "        "     16       26E8                                  38    18.50     "        "     10       25E8                                  39    26.20     "        "     4        27E8                                  40    40.00     "        "     1        23E8                                  41    0.00      1100     N.sub.2                                                                             40       16E9                                  42    0.10      "        "     41       15E9                                  43    0.20      "        "     40       16E9                                  44    0.80      "        "     35       16E9                                  45    1.60      "        "     29       17E9                                  46    3.60      "        "     24       17E9                                  47    13.20     "        "     16       17E9                                  48    18.50     "        "     10       16E9                                  49    26.20     "        "     5        17E9                                  50    40.00     "        "     1        17E9                                  51    0.00      1200     N.sub.2                                                                             92       23E9                                  52    0.10      "        "     92       24E9                                  53    0.20      "        "     90       25E9                                  54    0.80      "        "     87       26E9                                  55    1.60      "        "     81       25E9                                  56    3.60      "        "     67       26E9                                  57    13.20     "        "     32       26E9                                  58    18.50     "        "     21       23E9                                  59    26.20     "        "     6        25E9                                  60    40.00     "        "     1        27E9                                  61    0.00      1300     N.sub.2                                                                             92       30E9                                  62    0.10      "        "     92       34E9                                  63    0.20      "        "     90       34E9                                  64    0.80      "        "     87       35E9                                  65    1.60      "        "     81       35E9                                  66    3.60      "        "     67       36E9                                  67    13.20     "        "     32       37E9                                  68    18.50     "        "     21       36E9                                  69    26.20     "        "     6        36E9                                  70    40.00     "        "     1        35E9                                  71    0.00      1400     N.sub.2                                                                             92       33E9                                  72    0.10      "        "     92       34E9                                  73    0.20      "        "     90       34E9                                  74    0.80      "        "     87       35E9                                  75    1.60      "        "     82       35E9                                  76    3.60      "        "     67       36E9                                  77    13.20     "        "     31       37E9                                  78    18.50     "        "     20       36E9                                  79    26.20     "        "     6        36E9                                  80    40.00     "        "     1        35E9                                  81    0.00      1450     N.sub.2                                                                             92       36E9                                  82    0.10      "        "     92       37E9                                  83    0.20      "        "     90       36E9                                  84    0.80      "        "     87       37E9                                  85    1.60      "        "     81       37E9                                  86    3.60      "        "     66       36E9                                  87    13.20     "        "     32       37E9                                  88    18.50     "        "     20       36E9                                  89    26.20     "        "     6        37E9                                  90    40.00     "        "     1        37E9                                  91    0.00      1500     N.sub.2                                                                             86       40E9                                  92    0.10      "        "     85       42E9                                  93    0.20      "        "     82       40E9                                  94    0.80      "        "     77       41E9                                  95    1.60      "        "     70       42E9                                  96    3.60      "        "     65       41E9                                  97    13.20     "        "     25       43E9                                  98    18.50     "        "     16       43E9                                  99    26.20     "        "     3        41E9                                  100   40.00     "        "     1        40E9                                  ______________________________________                                    

                  TABLE 17                                                        ______________________________________                                        Firing Experiment Results (Hematite + Magnetite + Mg)                         of the Present Invention                                                            Amt. of Mg                                                                              Heating        Satn.    Grain                                 Sample                                                                              added     temp.    Atmos-                                                                              magnetization                                                                          strength                              No.   (wt %)    (°C.)                                                                           phere δs (emu/g)                                                                       (Pa)                                  ______________________________________                                        1     0.00      500      N.sub.2                                                                             60       11E6                                  2     0.10      "        "     61       12E6                                  3     0.20      "        "     61       15E6                                  4     0.80      "        "     55       12E6                                  5     1.60      "        "     49       11E6                                  6     3.60      "        "     37       11E6                                  7     13.20     "        "     26       15E6                                  8     18.50     "        "     20       12E6                                  9     26.20     "        "     7        14E6                                  10    40.00     "        "     3        14E6                                  11    0.00      550      N.sub.2                                                                             62       15E8                                  12    0.10      "        "     62       15E8                                  13    0.20      "        "     61       14E8                                  14    0.80      "        "     52       15E8                                  15    1.60      "        "     45       17E8                                  16    3.60      "        "     40       16E8                                  17    13.20     "        "     31       13E8                                  18    18.50     "        "     18       13E8                                  19    26.20     "        "     7        16E8                                  20    40.00     "        "     3        17E8                                  21    0.00      600      N.sub.2                                                                             62       17E8                                  22    0.10      "        "     62       15E8                                  23    0.20      "        "     61       16E8                                  24    0.80      "        "     52       14E8                                  25    1.60      "        "     44       15E8                                  26    3.60      "        "     40       17E8                                  27    13.20     "        "     31       15E8                                  28    18.50     "        "     20       15E8                                  29    26.20     "        "     6        14E8                                  30    40.00     "        "     3        15E8                                  31    0.00      800      N.sub.2                                                                             62       30E8                                  32    0.10      "        "     62       36E8                                  33    0.20      "        "     61       35E8                                  34    0.80      "        "     53       36E8                                  35    1.60      "        "     44       34E8                                  36    3.60      "        "     40       38E8                                  37    13.20     "        "     32       35E8                                  38    18.50     "        "     20       36E8                                  39    26.20     "        "     7        37E8                                  40    40.00     "        "     3        33E8                                  41    0.00      1100     N.sub.2                                                                             62       17E9                                  42    0.10      "        "     62       15E9                                  43    0.20      "        "     61       16E9                                  44    0.80      "        "     55       16E9                                  45    1.60      "        "     44       17E9                                  46    3.60      "        "     41       17E9                                  47    13.20     "        "     32       17E9                                  48    18.50     "        "     21       16E9                                  49    26.20     "        "     7        17E9                                  50    40.00     "        "     2        17E9                                  51    0.00      1200     N.sub.2                                                                             92       22E9                                  52    0.10      "        "     92       24E9                                  53    0.20      "        "     90       25E9                                  54    0.80      "        "     87       25E9                                  55    1.60      "        "     81       25E9                                  56    3.60      "        "     67       26E9                                  57    13.20     "        "     32       24E9                                  58    18.50     "        "     21       25E9                                  59    26.20     "        "     6        25E9                                  60    40.00     "        "     1        28E9                                  61    0.00      1300     N.sub.2                                                                             92       31E9                                  62    0.10      "        "     92       33E9                                  63    0.20      "        "     90       34E9                                  64    0.80      "        "     87       35E9                                  65    1.60      "        "     81       34E9                                  66    3.60      "        "     67       35E9                                  67    13.20     "        "     32       37E9                                  68    18.50     "        "     21       36E9                                  69    26.20     "        "     6        36E9                                  70    40.00     "        "     1        35E9                                  71    0.00      1400     N.sub.2                                                                             92       33E9                                  72    0.10      "        "     92       34E9                                  73    0.20      "        "     90       34E9                                  74    0.80      "        "     87       35E9                                  75    1.60      "        "     82       35E9                                  76    3.60      "        "     67       36E9                                  77    13.20     "        "     31       37E9                                  78    18.50     "        "     20       36E9                                  79    26.20     "        "     6        36E9                                  80    40.00     "        "     1        35E9                                  81    0.00      1450     N.sub.2                                                                             92       38E9                                  82    0.10      "        "     92       36E9                                  83    0.20      "        "     90       36E9                                  84    0.80      "        "     87       37E9                                  85    1.60      "        "     81       37E9                                  86    3.60      "        "     66       36E9                                  87    13.20     "        "     32       37E9                                  88    18.50     "        "     20       36E9                                  89    26.20     "        "     6        37E9                                  90    40.00     "        "     1        36E9                                  91    0.00      1500     N.sub.2                                                                             86       41E9                                  92    0.10      "        "     85       40E9                                  93    0.20      "        "     82       40E9                                  94    0.80      "        "     77       41E9                                  95    1.60      "        "     70       42E9                                  96    3.60      "        "     65       41E9                                  97    13.20     "        "     25       43E9                                  98    18.50     "        "     16       40E9                                  99    26.20     "        "     3        41E9                                  100   40.00     "        "     1        41E9                                  ______________________________________                                    

                  TABLE 18                                                        ______________________________________                                        Firing Experiment Results (Magnetite + Mg)                                    of the Present Invention                                                            Amt. of Mg                                                                              Heating        Satn.    Grain                                 Sample                                                                              added     temp.    Atmos-                                                                              magnetization                                                                          strength                              No.   (wt %)    (°C.)                                                                           phere δs (emu/g)                                                                       (Pa)                                  ______________________________________                                        1     0.00      500      N.sub.2                                                                             65       11E6                                  2     0.10      "        "     64       12E6                                  3     0.20      "        "     64       11E6                                  4     0.80      "        "     58       11E6                                  5     1.60      "        "     50       13E6                                  6     3.60      "        "     44       11E6                                  7     13.20     "        "     34       14E6                                  8     18.50     "        "     25       15E6                                  9     26.20     "        "     10       11E6                                  10    40.00     "        "     6        14E6                                  11    0.00      550      N.sub.2                                                                             92       15E8                                  12    0.10      "        "     92       15E8                                  13    0.20      "        "     91       16E8                                  14    0.80      "        "     89       15E8                                  15    1.60      "        "     82       18E8                                  16    3.60      "        "     72       16E8                                  17    13.20     "        "     34       17E8                                  18    18.50     "        "     25       17E8                                  19    26.20     "        "     10       16E8                                  20    40.00     "        "     6        17E8                                  21    0.00      600      N.sub.2                                                                             92       14E8                                  22    0.10      "        "     92       15E8                                  23    0.20      "        "     91       15E8                                  24    0.80      "        "     89       14E8                                  25    1.60      "        "     82       16E8                                  26    3.60      "        "     72       17E8                                  27    13.20     "        "     34       15E8                                  28    18.50     "        "     25       16E8                                  29    26.20     "        "     10       15E8                                  30    40.00     "        "     6        16E8                                  31    0.00      800      N.sub.2                                                                             92       22E8                                  32    0.10      "        "     92       26E8                                  33    0.20      "        "     91       25E8                                  34    0.80      "        "     88       26E8                                  35    1.60      "        "     82       24E8                                  36    3.60      "        "     72       26E8                                  37    13.20     "        "     33       25E8                                  38    18.50     "        "     25       28E8                                  39    26.20     "        "     11       27E8                                  40    40.00     "        "     6        23E8                                  41    0.00      1100     N.sub.2                                                                             92       17E9                                  42    0.10      "        "     92       15E9                                  43    0.20      "        "     91       16E9                                  44    0.80      "        "     88       16E9                                  45    1.60      "        "     83       18E9                                  46    3.60      "        "     72       17E9                                  47    13.20     "        "     33       17E9                                  48    18.50     "        "     25       17E9                                  49    26.20     "        "     11       17E9                                  50    40.00     "        "     7        17E9                                  51    0.00      1200     N.sub.2                                                                             92       23E9                                  52    0.10      "        "     92       24E9                                  53    0.20      "        "     90       26E9                                  54    0.80      "        "     87       25E9                                  55    1.60      "        "     81       26E9                                  56    3.60      "        "     67       27E9                                  57    13.20     "        "     33       24E9                                  58    18.50     "        "     21       25E9                                  59    26.20     "        "     7        25E9                                  60    40.00     "        "     1        28E9                                  61    0.00      1300     N.sub.2                                                                             92       35E9                                  62    0.10      "        "     92       33E9                                  63    0.20      "        "     90       34E9                                  64    0.80      "        "     87       35E9                                  65    1.60      "        "     81       34E9                                  66    3.60      "        "     65       35E9                                  67    13.20     "        "     32       37E9                                  68    18.50     "        "     21       35E9                                  69    26.20     "        "     6        36E9                                  70    40.00     "        "     1        35E9                                  71    0.00      1400     N.sub.2                                                                             92       33E9                                  72    0.10      "        "     92       34E9                                  73    0.20      "        "     90       34E9                                  74    0.80      "        "     87       35E9                                  75    1.60      "        "     82       35E9                                  76    3.60      "        "     65       36E9                                  77    13.20     "        "     31       37E9                                  78    18.50     "        "     20       37E9                                  79    26.20     "        "     6        36E9                                  80    40.00     "        "     1        35E9                                  81    0.00      1450     N.sub.2                                                                             92       38E9                                  82    0.10      "        "     92       36E9                                  83    0.20      "        "     90       36E9                                  84    0.80      "        "     87       37E9                                  85    1.60      "        "     81       36E9                                  86    3.60      "        "     66       36E9                                  87    13.20     "        "     33       36E9                                  88    18.50     "        "     21       37E9                                  89    26.20     "        "     6        38E9                                  90    40.00     "        "     1        37E9                                  91    0.00      1500     N.sub.2                                                                             86       40E9                                  92    0.10      "        "     85       40E9                                  93    0.20      "        "     82       40E9                                  94    0.80      "        "     76       41E9                                  95    1.60      "        "     70       42E9                                  96    3.60      "        "     65       41E9                                  97    13.20     "        "     23       43E9                                  98    18.50     "        "     16       40E9                                  99    26.20     "        "     2        41E9                                  100   40.00     "        "     1        41E9                                  ______________________________________                                    

                  TABLE 19                                                        ______________________________________                                        Firing Experiment Results (Hematite + Mg + Mn)                                of the Present Invention                                                                            Mg = 3.60 wt %                                                Amt. of Mn                                                                              Heating        Satn.    Grain                                 Sample                                                                              added     temp.    Atmos-                                                                              magnetization                                                                          strength                              No.   (wt %)    (°C.)                                                                           phere δs (emu/g)                                                                       (Pa)                                  ______________________________________                                        1     0.00       550     N.sub.2                                                                             27       14E8                                  2     0.10      "        "     27       14E8                                  3     0.20      "        "     27       14E8                                  4     0.50      "        "     28       16E8                                  5     1.00      "        "     29       17E8                                  6     2.00      "        "     31       17E8                                  7     3.00      "        "     32       17E8                                  8     4.00      "        "     33       17E8                                  9     6.00      "        "     35       17E8                                  10    8.00      "        "     37       17E8                                  11    10.00     "        "     39       17E8                                  12    0.00      1300     N.sub.2                                                                             67       36E9                                  13    0.10      "        "     67       36E9                                  14    0.20      "        "     67       37E9                                  15    0.50      "        "     68       40E9                                  16    1.00      "        "     69       45E9                                  17    2.00      "        "     71       48E9                                  18    3.00      "        "     72       50E9                                  19    4.00      "        "     73       51E9                                  20    6.00      "        "     75       52E9                                  21    8.00      "        "     77       52E9                                  22    10.00     "        "     80       52E9                                  ______________________________________                                    

We claim:
 1. A process for producing magnetic oxide, comprising the steps of mixing a given amount ranging from 0.20 to 76.0% by weight of at least one member selected from among Ca, Ti, Sn, Si and Mg magnetization control (regulation) materials with hematite, a mixture of hematite and magnetite, or magnetite as a starting material to obtain a mixed powder, adding 0.1 to 4.0% by weight of a liquid or powdery substance having a carbon-to-carbon single or double bond to the mixed powder, stirring the mixture to obtain a substantially homogeneous mixture and sintering the obtained mixture in an inert gas at 550° to 1500° C.
 2. The process according to claim 1, wherein said starting material is hematite or a mixture of hematite and magnetite, said magnetization control material is Ca and mixed in an amount of 1.43 to 64.0% by weight, and said sintering is conducted at 1200° to 1450° C.
 3. The process according to claim 1, wherein said starting material is magnetite, said magnetization control material is Ca and mixed in an amount of 1.43 to 64.0% by weight, and said sintering is conducted at 550° to 1450° C.
 4. The process according to claim 1, wherein, prior to said sintering, said mixed powder is granulated into globular grains to thereby render the powder globular.
 5. The process according to claim 1, wherein 47.3 to 98.8% by weight of magnetite as the starting material is mixed with 1.2 to 52.7% by weight in terms of Ti, of a Ti compound as the magnetization control material, and said sintering is conducted at 550° to 1450° C.
 6. The process according to claim 1, wherein a mixture of 9.8 to 98.8% by weight of magnetite and 0 to 79.0% by weight of hematite as the starting material is mixed with 1.2 to 52.7% by weight, in terms of Ti, of a Ti compound as the magnetization control material, and said sintering is conducted at 1200° to 1450° C.
 7. The process according to claim 1, wherein 24.0 to 99.2% by weight of hematite or a mixture of hematite and magnetite as the starting material is mixed with 0.8 to 76.0% by weight, in terms of Sn, of an Sn compound as the magnetization control material, and said sintering is conducted at 1200° to 1450° C.
 8. The process according to claim 1, wherein 24.0 to 99.2% by weight of magnetite as the starting material is mixed with 0.8 to 76.0% by weight, in terms of Sn, of an Sn compound as the magnetization control material, and said sintering is conducted at 550° to 1450° C.
 9. The process according to claim 1, wherein 35.0 to 99.5% by weight of hematite as the starting material is mixed with 0.5 to 65% by weight, in terms of Si, of an Si compound as the magnetization control material, and said sintering is conducted at 1200° to 1450° C.
 10. The process according to claim 1, wherein 35.0 to 99.5% by weight of a mixture of hematite and magnetite as the starting material is mixed with 0.5 to 65% by weight, in terms of Si, of an Si compound as the magnetization control material, and said sintering is conducted at 1200° to 1450° C.
 11. The process according to claim 1, wherein 35.0 to 99.5% by weight of magnetite as the starting material is mixed with 0.5 to 65% by weight, in terms of Si, of an Si compound as the magnetization control material, and said sintering is conducted at 550° to 1450° C.
 12. The process according to claim 1, wherein said starting material is hematite or a mixture of hematite and magnetite, the magnetization control material is an Mg compound and mixed in an amount of 0.20 to 18.50% by weight, in terms of Mg, and said sintering is conducted at 550° to 1500° C.
 13. The process according to claim 1, wherein said starting material is magnetite, the magnetization control material is an Mg compound and mixed in an amount of 0.20 to 26.20% by weight, in terms of Mg, and said sintering is conducted at 550° to 1500° C.
 14. The process according to claim 1, wherein said starting material is hematite or a mixture of hematite and magnetite compounded with 0.20 to 10.0% by weight of Mn, the magnetization control material is an Mg compound and mixed in an amount of 0.20 to 18.50% by weight, in terms of Mg, and said sintering is conducted at 550° to 1500° C.
 15. The process according to claim 1, wherein said starting material is magnetite compounded with 0.20 to 10.0% by weight of Mn, the magnetization control material is an Mg compound and mixed in an amount of 0.20 to 26.20% by weight, in terms of Mg, and said sintering is conducted at 550° to 1500° C.
 16. A magnetic oxide obtained by mixing a given amount ranging from 0.20 to 76.0% by weight of as least one member selected from among Ca, Ti, Sn, Si and Mg magnetization control (regulation) materials with hematite, a mixture of hematite and magnetite, or magnetite as a starting material to obtain a mixed powder, adding 0.1 to 4.0% by weight of a liquid or powdery substance having a carbon-to-carbon single or double bond to the mixed powder, stirring the mixture to obtain a substantially homogeneous mixture and sintering the obtained mixture in an inert gas at 550° to 1500° C. 